Title: Robotics and radiation hardening in the nuclear industry
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Title: Robotics and radiation hardening in the nuclear industry
Physical Description: Book
Language: English
Creator: Houssay, Laurent P., 1976-
Publisher: State University System of Florida
Place of Publication: Florida
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Publication Date: 2000
Copyright Date: 2000
 Subjects
Subject: Robotics   ( lcsh )
Nuclear and Radiological Engineering thesis, M.S   ( lcsh )
Dissertations, Academic -- Nuclear and Radiological Engineering -- UF   ( lcsh )
Genre: government publication (state, provincial, terriorial, dependent)   ( marcgt )
bibliography   ( marcgt )
theses   ( marcgt )
non-fiction   ( marcgt )
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Summary: ABSTRACT: A review of the robotic tools in the nuclear industry is presented. The complexity and efficiency of these systems has improved greatly during the last decade. However, the degradations induced by radiation often limit the capability of robotic systems and shorten their lifetime. The effects of radiation on robotic equipment are presented. The damages produced by radiation on electronic devices are particularly dramatic and are studied thoroughly. Several methods to upgrade the survivability of electronic circuits are developed. These techniques increase the lifetime of a robotic system but their development is often costly and time consuming. Finally, the importance of a good testing protocol is stressed. This guide is destined for engineers who are not familiar with robotic systems for nuclear applications and who need an overview of the impact of radiation on these systems.
Summary: KEYWORDS: nuclear, robotics, radiation, hardening, effects, testing, radiation hardening, radiation effects, gamma
Thesis: Thesis (M.S.)--University of Florida, 2000.
Bibliography: Includes bibliographical references (p. 186-197).
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System Details: Mode of access: World Wide Web.
Statement of Responsibility: by Laurent P. Houssay.
General Note: Title from first page of PDF file.
General Note: Document formatted into pages; contains xii, 198 p.; also contains graphics.
General Note: Vita.
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ROBOTICS AND RADIATION HARDENING IN THE NUCLEAR INDUSTRY


By

LAURENT P. HOUSSAY












A THESIS PRESENTED TO THE GRADUATE SCHOOL
OF THE UNIVERSITY OF FLORIDA IN PARTIAL FULFILLMENT
OF THE REQUIREMENTS FOR THE DEGREE OF
MASTER OF SCIENCE

UNIVERSITY OF FLORIDA


2000

































Copyright 2000

by

Laurent P. Houssay



































To my Parents















ACKNOWLEDGMENTS


My sincere thanks go to the members of my supervisory committee, Chairman

Professor James S. Tulenko, Dr. G. Ronald Dalton and James L. Kurtz, for their guidance

and advice.

Special thanks go to Mu Yang for his constant support. Profitable discussion and

dosimetry data from Dr. Georgi Georgiev are acknowledged and appreciated. Thanks

also go to the members of the Electrical Communication Lab, Jason M. Cowdery, Charles

Overman and Bruce Horton for their valuable help and advice. Corrections provided by

Denielle Caparco were very helpful and appreciated.

The work on this thesis project was as a part of the U.S. Department of Energy

sponsored program in Applications of Robotics for Hazardous Environments. This

program supplied much of the funding to complete this work.
















TABLE OF CONTENTS


page

A C K N O W L E D G M E N T S ........................................................................ .................... iv

LIST OF TABLES ................................. ........... ..................................... viii

L IST O F FIG U R E S .................................................................... .. ........ ..........x....x

A B S T R A C T ...................................................................................................................... x ii



1 OPERATIONAL ENVIRONMENTS IN THE NUCLEAR INDUSTRY..................... 1

F u el F ab ric atio n .............................................................................................................. 1
R actor Sy stem O operation ................................................................. .. ...................... 2
Spent Fuel Handling and Storage In the Power Plant............................................... 4
Spent Fuel Disassembly and W aste Processing.......................................... .............. 4
W aste H handling and Storage ......................................................................... .............. 5
D econtam nation and D ecom m issioning .................................................... .............. 6


2 USE OF ROBOTIC SYSTEMS IN THE NUCLEAR INDUSTRY...........................9...

N eed for Robotics System s .............. ...... .......... .............. 9
Mobile Robots for Routine Monitoring and Surveillance ...................................... 10
Inspection and Maintenance of Reactor Components............................................. 13
Inspection and Cleaning of a Typical Steam Generator...................................... 13
Inspection of R actor V essel................................... ........................ .............. 20
P ip e In sp ectio n .......................................................................................................... 2 4
U nderw ater Inspection... ................................................................. .............. 25
H handling and Processing of W aste ........................................................... .............. 28
D econtam nation and D ecom m issioning .................................................. .............. 30
Su rface C lean u p ........................................................................................................ 3 0
T an k C lean u p ............................................................................................................ 3 3
Decommissioning......................... ............ .................................... 43
P ost A accident O operation ... ..................................................................... .............. 48




v









3 R A D IA TIO N EFFEC T S .......................................................................... ................ 51

Definition and Units in Nuclear Engineering............................................ .............. 51
R a d io a ctiv ity ............................................................................................................. 5 1
A activity ............................. .. ...................... ....... .............. 51
D ecay constant, M ean-life, H alf-life..................................................... .............. 52
E n erg y ....................................................................................................... . ........... 5 2
D o sim etry .................... .... ................................................................................. .. 5 2
Types of R radiation and their Interaction................................................... .............. 53
Photons: G am m a and X -rays .................................. ........................ .............. 53
B eta: E lectron and P ositron........................................ ........................ .............. 55
H eavy C charged Particle .. ................................................................... .............. 56
N e u tro n ...................................................................................................................... 5 7
C o n c lu sio n ................... ..... .................................................................................... 5 8
Radiation Effects on Passive Elem ents ....................... ......................................... 59
Inorganic Materials ............................. ......... ....................... 59
O organic M materials ... .. ... .............. ................................................ ...... ....... .. 6 1
O p tical M material ........................................................................................................ 6 5
Electronic and Electrical Com ponents .................................................. .............. 68
Mechanical and Electromechanical Components ............................................... 74
R radiation Effects on Sem iconductors....................................................... .............. 75
Physical Effects on Sem iconductors .................................................... .............. 76
T technology F am ilies ..... .. ........................................ ........................ . . ........ .. 80
D iscrete C om pon ents ....... .. ...................................... ........................ .............. 96
O p to electro n ics ......................................................................................................... 9 9
D igital Integrated C ircuits .................................... ......................... .............. 102
A nalog Integrated C ircuits .................................... ........................ .............. 104


4 RADIATION HARDENING TECHNIQUES ......................................................107

Definition of Failure.............................. .......... ....................... 107
Minimal Approach......................... ............ .................................... 108
"Split" Technique ...................................... .......... ....................... 108
M maintenance and R epairs ..................................... ......................... .............. 109
S h ie ld in g ............................................................................................................ . . 1 1 0
R radiation H gardening Strategy................................... ........................ .............. 112
Modification of an Existing Design...... ........ ...... .................... 112
Innovative Design ............. .......................................... .............. 113
Radiation-hardened Components ...... .............. .............. .................... 114
D efin ition .............................................................................................. .. . .......... 1 14
Presentation of Radiation-hardened Components ............................................. 115
Advantages and Limitations of Radiation-hardened Components....................... 116
Use of Commercial Off-The-Shelf Component...... .... .................................. 118
Radiation Hardening Design Technique .............. .................... 121
Selection of C om ponents ..................................... ......................... .............. 12 1
R radiation Tolerant D esign .................................... ........................ .............. 122









A nnealing ........................................................................................................ 123
Biasing .................................................. ...... .............. 123


5 R AD IA TION TESTIN G ................................................................. ................ 125

Radiation Types and Testing Facilities...... .... .... .................... 125
C obalt-60 ................................................................................... ...................... 125
S p e n t F u e l ........................... ...... ............................................................................ 1 2 7
X-ray M machines and Particle Accelerators ...... .......... .................................... 128
Testing Conditions ........... ... ................... ......... .............. ............ .. 129
D ose R ate ....................................................................................................... 129
T e m p e ratu re ............................................................................................................ 1 3 1
Biasing ................................................... ... .............. 132
Other Parameters ........ .. .............................. ......... ......... ............ .. 133
Dosimetry .................................................................... 133
D ose R ate D etectors..... .. .................................. ........................... ............ .. 133
Total D ose Sensors..... .. ................................... ........................... ............ .. 134
T e stin g P ro c e d u re ........................................................................................................ 13 6
N um ber of Sam ples ..... .. ........................................ ........................ ............ .. 136
Testing Conditions ...................................... ......... ....................... 136
T testing E quipm ent ....... .. ....................................... ......................... . . ........ .. 137
Choice of Test Param eters. ................ ......................................................... 138
N o rm s ...................................................................................................................... 1 3 8


6 C O N C L U SIO N ........................................................................................................... 14 0

APPENDIX A: COMPARISON OF LEAD AND TUNGSTEN SHIELD...................145

APPENDIX B: TOTAL DOSE TESTING OF A GaAs AMPLIFIER..........................147

APPENDIX C: TOTAL DOSE TESTING OF A BANDPASS FILTER......................158

APPENDIX D: TOTAL DOSE TESTING OF AN OPERATIONAL AMPLIFIER ...... 168

APPENDIX E: TOTAL DOSE TESTING OF A GALLIUM ARSENIDE MIXER ...... 177

L IST O F R E FE R E N C E S ................................................................................................. 186

BIOGRAPH ICAL SKETCH .................. .............................................................. 198















LIST OF TABLES



Table Page

1: Radiation environment in a CAGR nuclear reactor ............... ................................. 3

2: Radiation environment around a PWR on load ........................................................... 3

3: Radiation environment around a PWR during refueling.............................................. 3

4: D ose rate during fueling operation...................................... ......................... ............. 4

5: Nuclear environments associated with waste processing............................................. 5

6: Typical dose rate of various medium and high level wastes ....................................... 6

7: Dose rate for typical CAGR decommissioning tasks................................................... 7

8: Typical decommissioning environments of a PWR ............... ................................. 7

9: Photon tenth value thickness in cm for Al, Fe, Pb and concrete...................................54

10: R ange of electron in alum inum ........................................ ........................ ................ 56

11: Range ([tm) of alpha particles in Al, Pb, water and air ..............................................57

12: R ange of protons in alum inum ......................................... ........................ ................ 57

13: Radiation damage thresholds on metals.....................................................60

14: R radiation dam age thresholds on ceram ics.................................................. ................ 60

15: R radiation tolerance of plastics ......................................... ........................ ................ 62

16: R radiation dam ages on coatings........................................ ........................ ................ 63

17: R radiation dam ages on adhesives ...................................... ....................... ................ 64

18: R radiation effects on lubricants......................................... ........................ ................ 65

19: R radiation dam ages on w indow glasses....................................................... ................ 66


viii









20: R radiation dam ages thresholds on resistors ................................................. ................ 69

21: R radiation dam ages on capacitors....................................... ...................... ................ 70

22: Radiation damages on connectors, switches and relays..............................................73

A-1: Comparison of lead and tungsten shields .......... ......................... 146

B -1: O utput data versus total dose ...................................... ........................ ................ 156

C-1: Results of the bandpass filter irradiation .......... ..........................166

D-1 :Evolution of the DC offset with the total dose. ........................................ ................ 176















LIST OF FIGURES



Figure Page

1: Internal structure of a steam generator................................... ...................... ............. 14

2: Trapping zones at Si-SiO 2 interface..................................... ........................ ............. 79

3: Radiation effects on n-channel M OS devices................................................. 86

4: Effect of biasing on threshold voltage of MOS devices ............................................. 90

5: Attenuation provided by lead and tungsten shields........................................................ 111

6: Radiation tolerance by families of components....... ... ........................................ 119

B-1: Dose Map of the University of Florida Cobalt Irradiation................. ...................149

B-2: GaA s am plifier test configuration....................................................... ................ 151

B -3: G aA s am plifier testing board...................................... ........................ ............... 152

B-4: University of Florida Cobalt Irradiation facility ....... ... ...................................... 153

B-5: GaAs amplifier in the irradiation chamber .......... .........................154

B-6: Window of the testing at the end of the experiment ......................... ...................155

B-7: Variations of power with total dose....... ......... ......... ..................... 156

B-8: Variations of magnitude with total dose.............. ......................157

C-1: Dose Map of the University of Florida Cobalt Irradiation................. .................. 160

C-2: Bandpass filter prepared for a radiation test....... ........... ........................................ 162

C -3 : B andpass fi lter circuit.................................................. ............................................ 163

C-4: B andpass filter test configuration........................................................ ............... 163

C-5: University of Florida Cobalt Irradiation facility ....... ... ....................................... 164









C-6: The bandpass filter in the irradiation chamber...... ......... ....................................... 165

C-7: Results of the bandpass filter irradiation .......... ..........................167

D-1: Dose Map of the University of Florida Cobalt Irradiation................. ...................170

D-2: Operational amplifier testing board...... .......... ......... ..................... 171

D-3: Operational amplifier test configuration................ ..........................172

D-4: University of Florida Cobalt Irradiation facility...... .... ..................................... 173

D-5: Operational amplifier in the irradiation chamber ..................................... ................ 174

D-6: Evolution of the frequency response with the total dose.................... ...................175

D-7: Evolution of the DC offset with the total dose ....... ... ...................................... 176

E-1: Dose Map of the University of Florida Cobalt Irradiation.................. ...................179

E-2: G aA s m ixer testing board. ................................................................... ............... 180

E-3: G aA s m ixer test configuration ..................................... ....................... ............... 181

E-4: Testing setup in the irradiation room ....... ........ ....... ...................... 181

E-5: University of Florida Cobalt Irradiation facility ....... ... ..................................... 182

E-6: The G aA s m ixer testing board ................. ........................................................... 183

E-7: Evolution of the amplitude versus total dose ....... ........... ....................................... 184

E-8: Evolution of the phase difference versus total dose...... .................... .................. 185















Abstract of Thesis Presented to the Graduate School
of the University of Florida in Partial Fulfillment of the
Requirements for the Degree of Master of Science

ROBOTICS AND RADIATION HARDENING IN THE
NUCLEAR INDUSTRY

By

Laurent P. Houssay

August 2000


Chairman: Professor James S. Tulenko
Major Department: Nuclear and Radiological Engineering

A review of the robotic tools in the nuclear industry is presented. The complexity

and efficiency of these systems has improved greatly during the last decade. However,

the degradations induced by radiation often limit the capability of robotic systems and

shorten their lifetime. The effects of radiation on robotic equipment are presented. The

damages produced by radiation on electronic devices are particularly dramatic and are

studied thoroughly. Several methods to upgrade the survivability of electronic circuits

are developed. These techniques increase the lifetime of a robotic system but their

development is often costly and time consuming. Finally, the importance of a good

testing protocol is stressed. This guide is destined for engineers who are not familiar with

robotic systems for nuclear applications and who need an overview of the impact of

radiation on these systems.














CHAPTER 1
OPERATIONAL ENVIRONMENTS IN THE NUCLEAR INDUSTRY



Not only is radiation hazardous to humans but it is hazardous to electronics as

well. When designing a system for radiation environments, the total dose, the type of

radiation and sometimes the dose rates are major factors that will limit the lifetime and

the reliability of the equipment. In nuclear science, there is radiation in every step of the

fuel cycle. The real concern is for the spent fuel and the decommissioning of nuclear

facilities where the highest radiation fields are found. The data presented in this section

comes from an evaluation made in English facilities [1] and from the RADECS 99

(RADiation Effects on Components and Systems) technical book [2-3]. This data gives

the dose rate in several facilities for typical work on the equipment. The following dose

rates are only meant to indicate an order of magnitude. This information will allow

engineers to calculate the total dose requirement for the system over its expected lifetime.

All indicated dose rates are referenced to energy deposited in silicon.




Fuel Fabrication


Due to the wide variety of nuclear reactor fuels it is very difficult to give an

accurate estimate of the dose rate in the fuel fabrication process. It is known, however,

that mixed oxide fuel (Uranium + Plutonium) have higher radiation levels when

compared to uranium based fuels. This is due to the plutonium that has its own activity









and to small concentration of radioactive impurities. The main source of radiation of new

fuel is low energy gamma rays. The penetrating power of these photons is weak and the

dose rate is at its highest when the fuel is in powder form. The photon dose rate ranges

from 10-4 Gy/h to 10-1 Gy/h with a common value is 10-3 Gy/h. The contribution of

neutrons that come from the decay of transuranics associated with plutonium is much

weaker and is between 10-6 Gy/h and 10-4 Gy/h. In most cases the effects of radiation on

robotics from new fuel elements are close to zero.




Reactor System Operation


The dose rates existing in the core of a fission reactor are the highest dose rates

associated with nuclear power production. It is also a place where the neutron dose rate

causes significant damage to electronics. The neutron flux is so high that it creates single

event upsets, in much the same manner as cosmic rays in space. The equipment that can

be used in such an environment must be highly hardened against radiation damages.

Table 1 indicates the gamma and neutron dose rates when the reactor is on load and

shutdown. The values of dose rate are typical for a commercial advanced gas cooled

reactor (CAGR), and are close to the ones from a pressurized water reactor (PWR).









Table 1: Radiation environment in a CAGR nuclear reactor
Location State of the Gamma dose Neutron dose Neutron flux
reactor rate (Gy/h) rate (Gy/h) (n/cm2/h)
On load 107 107 1017
With4 the core0
Within the core Shut-down 104 5. 10-1 1010

Outside the On load 102 10 5. 1010
radial shield Shut-down 10-3 Negligible 104
Above the On load 10 1 1010
pressure dome Shut-down 5. 10-3 Negligible 10
Coolant loop On load 5. 10-1 Negligible Negligible
area
Above On load 5. 10-4 3. 10-4 Negligible
operating deck.
Source of data: see references [1, 2, 3]

The following Tables 2 and 3 indicate additional typical dose rate values for a

PWR on load and during refueling/maintenance operation.




Table 2: Radiation environment around a PWR on load
Location Gamma dose rate (Gy/h) Neutron dose rate (Gy/h)
Pressure vessel annulus 102 3. 102
Coolant loop area 5. 10-1
Outside the loop area 2. 10-3 2. 10-4
Source of data: see references [1, 2, 3]


Table 3: Radiation environment around a PWR during refueling
Location Gamma dose rate (Gy/h)
Lower intervals of the pressure vessel 2. 10-1
Upper intervals of the pressure vessel 7. 10-2
Neutron shield pad >103
Around the steam generators 1.5 10-1
Coolant loop area 3. 10-3
Source of data: see references [1, 3]

When the reactor is on load, the gamma spectrum consists mainly of 1 to 5 MeV

photons resulting from Nitrogen-16 decay. The energy is weaker (between 0.3 and 1.3









MeV) when the reactor is shut down, with the major contributor being Cobalt-60 from

the activated steel structures.




Spent Fuel Handling and Storage In the Power Plant


In many reactors, fuel elements are removed from the reactor and placed in a fuel

storage pool. The fuel is stored there until its elements with the shortest half-life have

decayed. After their activity has significantly decreased, the fuel elements are shipped to

a waste processing plant.




Table 4: Dose rate during fueling operation
Gamma dose rate Neutron dose rate Neutron flux
(Gy/h) (Gy/h) (n/cm2/h)

Within the fuelling 5 102 From 1011 to 1012
machine near the fuel
From 103 to 104
In the storage pond rom 10 to 10 Negligible Negligible
near the fuel
Source of data: see references [1, 2, 3]



Spent Fuel Disassembly and Waste Processing


Waste processing activity consists of many steps and offers a wide range of

radiation levels. It is impossible to have a generic estimate of dose rates because each

facility, technology and type of fuel will have different results. Nevertheless, it is known

that the gamma rays give the highest contribution to the dose. Mechanical stripping and

cutting of the fuel elements generates about 103 Gy/h. Chemical processing is less

aggressive with 102 Gy/h. Vitrification of high-level waste is the final step and generates









up to 104 Gy/h, which is extremely high. Fuel reprocessing is often executed in a very

harsh environment and the radiation is only one of the many aggressive factors. Due to

high dose rates and the abundance of remote operations, waste processing employs the

greatest array of robotic systems in the entire nuclear industry. The following Table

describes several waste processing environments.




Table 5: Nuclear environments associated with waste processing
Maximum gamma dose rate Maximum neutron dose rate
Operation (Gy/h) (Gy/h)
CAGR dismantling facility 103 10-3
Storage pond 103 10-3
Shearing 103 10-3
Dissolution 103 10-3
Solvent extraction 102 10-3
Plutonium finishing 2. 10-2 4. 10-4
Uranium finishing 5. 10-3 4. 10-4
Vitrification 104 4. 10-4
Source of data: see references [1, 3]



Waste Handling and Storage.


There are two categories of waste: high and low-level wastes. High-level waste is

spent fuel that has been processed and vitrified. The risks involved with this kind of

waste are significant due to the hazardous compounds that are concentrated. For

example, the gamma dose rate at a distance of 1 meter from an unshielded vitrified

element is up to 200 Gy/h. The storage of high-level waste is a major undertaking that is

governed by many regulations. A complex setup of shielding, monitoring and remote

handling is required since the activity of this type of radioactive material is extremely









high. Each equipment requirement should be linked to the expected work environment,

because a wide range of dose rates can be found.

Low-level waste is less of a threat to robotics and to the health of workers. This

waste is composed of lightly contaminated waste such as gloves or concrete from

decommissioning. The contaminants are radioactive elements with low activity and short

half-life. Low-level waste is usually contained in metallic drums and stored in protected

sites on surfaces or superficially buried. Dose rates associated with low-level waste do

not create damage to electronics most of the time. Unlike high-level waste, lighter

shielding and packaging systems are required and used to reduce the exposure to workers.

The next Table indicates several dose rates at the surface and at 1 m from various nuclear

wastes.




Table 6: Typical dose rate of various medium and high level wastes
Dose rate at 1 m distance in
Waste description Dose rate at surface (Gy/h) air (Gy/h)
air (Gy/h)
Medium level sludge 6 4.5 10-1
Cemented waste 2. 10-3 10-4
High level vitrified 1.8 103 2. 102
Cemented resins (ions Up to 3. 10-1 Up to 1.5 1-2
exhngr)Up to 3. 10 Up to 1.5 10
exchangers)
Filter elements in concrete Up to 1. 10-1 Up to 5. 10-3
Miscellaneous Up to 5. 10-3 Up to 2. 10-4
Source of data: see references [3]



Decontamination and Decommissioning.


Many nuclear power plants as well as "cold war" nuclear weapons production

facilities built in the 1950s, have reached the end of their lifetime. These buildings









require decontamination, which can be accomplished by cleaning up the radioactive

contamination and then decommissioning the facility. The dose rate at 1 cm from 2

grams of spent fuel is on the order of 10 Gy/h. Certain parts of the equipment have been

activated by the neutron flux over the years. These parts of the reactor are less and less

activated as the distance from the core increases. The next Tables indicate typical doses

rate associated with the decommissioning of a CAGR (Commercial Advanced Gas

Cooled Reactor) and PWR (Pressurized Water Reactor) (Mol Belgium).


Table 7: Dose rate for typical CAGR decommissioning tasks
Location Gamma dose rate (Gy/h)
Debris vault 50
In-reactor steel components 1 to 10
Graphite stack 3. 10-2
Diagrid region 2. 10-4 to 2. 10-3
Boiler region 10-4 to 10-3
Vessel concrete 10-4
Standpipe region 10-4 to 10-3
Source of data: see references [1, 3]


Table 8: Typical decommissioning environments of a PWR
Location Dose rate (Gy/h)
De-activation pool 10-5
Reactor pool 2. 10-5
Reactor building 2. 105
Shipping area 2. 105
Primary circuit 4. 10-1 to 1.2 10-2
Building hot spots 2 to 1.2 10-2
Shipping area near waste drums 2 to 3. 10-1
De-activation pool near fuel 101 to 103
Vessel mid plane 102 to 103
Source of data: see references [3]






8


There are currently many storage tanks in the US that require clean up due to

leaks and structural problems. The liquids and sludge contained in these tanks are highly

radioactive. For example, the Department of Energy (DOE) owns the Hanford storage

tanks, which contain 232,000 cubic meters of mixed hazardous waste, with a total activity

of 9.25 1018 Becquerel (2.5 108 Curies) [4]. This means that one-gallon generates an

activity of 4 Curies. For safety and access reasons, robotic tools are the only safe means

to cleanup such facilities.














CHAPTER 2
USE OF ROBOTIC SYSTEMS IN THE NUCLEAR INDUSTRY




Need for Robotics Systems


Today robots are widely used in the nuclear industry. Their main application is to

perform automated and repetitive work or to execute hazardous tasks that are dangerous

to human beings. First, profitability is the motivation to switch from a regular worker to

an automated system. Second, safety of the worker and regulation are issues that should

not be ignored. In nuclear science, protection of workers became a catalyst for the

development of robotics. Today, the regulation 10 CFR 20 from the Nuclear Regulatory

Commission indicates that an occupational worker cannot receive more than 50 mSv per

year for the full body dose [5]. Due to this dose regulation more workers have to be

employed to accomplish a mission. Once the maximum dose has been reached the

employee must stop working immediately. A study published in 1987 [6] indicates that

worker exposure costs more than $500,000 per man-Sv. On the contrary, a utility

executive said in 1990 that every dollar spent on robotics is doubled in return [7]. In

nuclear power plants, the reactor must be shut down or at least be brought to a fraction of

its maximum power to allow human intervention near the core. Robots shorten the

maintenance time and the number of workers needed; this reduction generates many

additional savings: less protective clothes, waste and paperwork. Due to the aging of

nuclear reactors, increased inspections and repairs are needed at a deeper level than ever









before. Finally, an estimation of the hazardous environment in an emergency situation

avoids putting personnel at risk. Although human safety is the main reason for the use of

robots, profitability is also a strong motivator. Furthermore, a remote system is often the

only way to enter a very high radiation field. The use of robotics in the nuclear industry

is obvious due to the factors mentioned.

This chapter is dedicated to the discussion of several robots that were designed or

used in the nineties. In most cases the robots are designed for "exceptional use," which

means that they are not running continuously but rather only when they are needed.

Inspection and repair robots are a good example of such tasks. Since these systems are

not permanently kept in a radiation field, their hardening is a minor concern compared to

their task efficiency. In contrast, a robot whose aim is to manipulate radioactive

equipment on a daily basis has a much greater need for radiation-tolerant components.




Mobile Robots for Routine Monitoring and Surveillance


The SIMON (Semi-Intelligent Mobile Observing Navigator) robot is a mobile

monitoring and surveillance robot developed in 1990 and introduced to the DOE's

Savannah River site the same year [8]. There were three requirements present in the

design of SIMON to avoid the need for human inspection of this nuclear facility. These

requirements were to measure radiation, to measure temperature and to retransmit

televised views of the area. The results were a great success in that SIMON located 20

spots of beta contamination that were undetected by "human" surveys [9]. SIMON is

equipped with the three-wheeled base of the Cybermotion K2A robot. Equipment used

with SIMON includes radiation detectors, temperature sensors and a camera mounted on









a telescoping mast. The positioning equipment consists of optical encoders on the motor

and the wheels, infrared beams and ultrasonic pulses to determine the position relative to

its docking station, and sonar and bumpers are used for collision avoidance. SIMON

navigates into a room by either following a preprogrammed path or it is controlled

manually to perform its monitoring task. It returns to the dock to recharge its batteries

when necessary. A computer program called "Dispatcher" choreographs the

preprogrammed path. Ultra-high frequency radio communication (UHF) links the host

computer to the robot. Once the "Dispatcher" is downloaded, SIMON can still run if the

radio link is out of range. Three on-board computers manage the resources of the robot.

The electronic components that are used in SIMON were chosen because of their natural

radiation resistance. A total dose radiation hardness of 200 Gy (20 krad) is achieved at a

reasonable cost using this design approach. The reliability is also enhanced by a self-

diagnostic capability. SIMON received a US patent in 1994.

The robot MACS (Mobile Automated Characterization System) was designed in

1996 as the second generation of SIMON robots [9]. This time the goal was to develop a

contamination map of nuclear facilities for decontamination and decommissioning

purposes. This task is performed automatically and in a reliable manner as compared to

workers equipped with portable detectors. MACS is equipped with highly sensitive

detectors and transmits the dose information in real time to the host station. The data

measured is incorporated into an interface called RadMap, which generates a color-coded

map of the contamination on the floor.

The robot ARIES (Autonomous Robotic Inspection Experimental System) was a

robot developed by the University of South Carolina for the DOE (Department of









Energy) [10]. The DOE stores several thousands of steel drums that contain low-level

radioactive waste. The drums are stacked and stored in long aisles. A weekly inspection

of the packaging integrity is required. This work is tedious and long-term exposure must

be avoided, although the radiation level is not hazardous to workers. The goal of ARIES

is to locate each drum and to perform a visual inspection to find paint blisters, rusted

areas or any other sign of container degradation. Since a bar code is located on every

drum, a database containing all the drum characteristics can be updated consistently and

regularly. When a damaged drum is found, its contents are repacked. ARIES was

developed after the SWAMI robot (Stored Waste Autonomous Mobile Inspector) [11].

SWAMI was designed at the DOE Savannah River site for the same application as the

ARIES robot. The two robots have fairly similar capabilities, however ARIES is more

modern and more sophisticated. ARIES uses the K3A mobile platform manufactured by

Cybermotion. This platform, associated with an elaborate sonar and light navigation

system, allows the robot to navigate in a narrow aisle of stacks of drums. The camera

positioning system (CPS) is a module that characterizes each drum. The module includes

a camera, a bar code scanner and a strobe light. Whenever ARIES arrives in front of a

drum stack, the CPS extends up and takes two pictures of the drum. The on-board

computer then processes the images and updates the database while the module moves up

to the next drum. Once the stack is completely inspected, the robot moves to the next

stack or to its dock to recharge its batteries. The system has been successfully tested in

several DOE facilities including the Fernald site and INEEL (Idaho National Engineering

and Environmental Laboratory). A radiation hardening study concluded that a radiation-

hardened version of ARIES was not needed since the radiation level in the storage facility









was too low to jeopardize the reliability of the equipment. The major shortcoming of the

present system is its low drum survey speed, which allows inspection of only one-third of

the expected 10,000 drums per week, due to battery problems [12].




Inspection and Maintenance of Reactor Components



Inspection and Cleaning of a Typical Steam Generator


The degradation of the steam generator performance is a technical challenge both

on the secondary and the primary side. In a steam generator, the primary hot water from

the reactor enters the generator at its base and moves to the other side through a bundle of

tubes. The tubes heat the secondary water that fills the vessel and then generates steam.

Figure 1 shows the internal structure of a steam generator. The sludge that builds up on

the secondary side of steam generators reduces its thermal efficiency. This accumulation

decreases the power output of the power plant and can cost millions of dollars in lost

revenues. Lower thermal efficiency is not the only consequence of the sludge. Another

major problem is the corrosion generated by the chemical elements that make up the

sludge, which may lead to cracks in the tubes [13]. Fretting leaks due to the presence of

foreign objects may also require the complete replacement of a steam generator. The

major challenges of the sludge removal are the extremely difficult access inside the steam

generator and the radiation field existing in this area. One cleaning method for removing

the sludge is chemical cleaning. Unfortunately, this process is expensive and generates

mixed chemical plus radioactive wastes that are difficult to store. A widely used cleaning









technique consists of highly effective water jets that are powerful enough to remove soft

as well as hard sludge.


Support plates













Divider plate

Primary coolant inlet


Steam

t t'M t


Tube









SSecondary water


Tubesheet

Manway
- Primary coolant outlet


Primary hot water Primary cold water


Figure 1: Internal structure of a steam generator



The robot CECIL (Consolidated Edison Combined Inspection and Lancing) is

designed for steam generator maintenance [14]. It is limited to cleaning the bottom part

of the steam generator, between the tubesheet and the first support plate. Michael

Reinhardt [15] provides an clear description of the CECIL setup, "CECIL 4 rides on a rail









installed through the steam generator inspection port. The robot can move longitudinally

along the rail assembly and extend and retract a flexible lance. This lance can be driven

between the columns of the U-tubes, and the robot can rotate about its axis to position the

lance at any point within the tube bundle above the tubesheet and below the first support

plate. Water jets are introduced into the tube bundle by way of these flex lances and the

robot's barrel sprays. The flex lances are the transport mechanism for inserting spray

nozzles and a video probe deep into the tube bundle." The system is remotely controlled

by a workstation installed in a low radiation zone. The operator can monitor and control

the position of the robot and the lance in real time. The water used in the process is

continuously pumped and processed in order to reduce waste volume. Several types of

lances are available, each one for a different task. One of the most interesting group of

lances is not used for cleaning the sludge but to locate and remove small objects

accidentally lost deep in the tube bundle. These objects can create significant damage

leading to fretting leakage and must be removed. The FOSAR (Foreign Object Search

and Retrieval) lance is equipped with one of four grappling tools to retrieve these objects

[16]. This system has been demonstrated successfully several times [17]. The first

cleaning test of CECIL 4 was carried out at Indian Point 2 in 1989. Later in 1991, the

FOSAR system removed a weld rod deep inside a steam generator at the Salem-1 reactor

[18].

The robots called UBIB (Upper Bundle in Bundle) and UBHC (Upper Bundle

Hydraulic Cleaning) were introduced in 1996 [19]. These robots offer a considerably

increased cleaning and inspection efficiency compared to the CECIL system. The

CECIL robot is limited to the region between the tube sheet and the first support plate of









the steam generator, whereas the UBIB and UBHC robots can access the upper bundle

region without drilling holes in the shell. UBIB is the upper bundle in the bundle

inspection system that is dedicated to remote visual inspection of steam generators. The

UBIB system is installed, like CECIL, through the hand hole at the bottom of the steam

generator, and its mast is extended to the upper region. A video camera placed at the end

of a wand can then inspect the bundle. A complete inspection of a steam generator can

be completed in one day and provides information for the cleaning process. The UBHC

robot is the cleaning version of the UBIB robot. UBHC is deployed the same way as

UBIC but is equipped with 20 nozzles in addition to a camera and lighting system. These

powerful water jets deliver 3000 psi water at a rate of 70 gallons-per-minute while the

position of the equipment is secured by inflating support bladders. The complete

cleaning operation lasts two days or even less if a previous inspection by UBIB did not

require a full bundle cleaning [20]. An operator located in a control station located in a

low dose area remotely controls the inspection and cleaning operations. The inside of the

steam generator and the location of the robot are monitored in real time. The system also

provides many safety features that stop the process in case of problems. Both the UHBC

and UBIB systems are currently designed to treat Westinghouse Model 44, 51 and F

steam generators. UBHC has been commercialized since February 1999 and has

successfully completed numerous cleaning operations in more than four countries.

A study assessing the occupational radiation exposure in European nuclear

reactors [21] indicates that the highest doses experienced in a PWR (Pressurized Water

Reactor) are for steam generator primary side work, where the median exposure value

exceeds 200 man-mSv. Since the maintenance is performed remotely, the majority of the









worker exposure occurs during the installation of the inspection and cleaning equipment.

Therefore the total dose received by the workers is primarily proportional to the number

of steam generators opened rather than on the duration of the maintenance. The major

contribution to the exposure during maintenance comes from the installation and the

removal of the nozzle dams in the steam generator. The dose rate in the steam generator

bowls is high: from 50 mSv/h to 250 mSv/h. Nozzle dams are installed in the inlet and

outlet nozzle of the steam generator to isolate the primary loop that is flooded during

refueling. The replacement of the workers by a robot for this specific task considerably

decreases the personnel exposure. At least two systems have been designed to meet this

challenge. Both systems use a remotely controlled robot positioned in the channel head.

With the oldest design, several cameras and lights are mounted on the robot, which

allows the operator to control a manipulator arm very accurately. The robot is located

outside the steam generator and operates through a man-way. The nozzle dam is then

positioned carefully by the arm and secured with bolts. This method has been

successfully tested with several handling robots: with a Schilling hydraulic manipulator

in 1991 [22], with the ROMA (Remotely Operated Manipulator Arm) robot in 1990 and

the ROSA III robot in 1992 [23]. The need of a handling robot has become obsolete,

thanks to the system developed, tested and designed by Foster-Miller [24]. A robotic arm

is avoided, because movement of a bulky robot at the bottom of the steam generator is

difficult and because the complexity of the installation requires highly trained personnel.

The idea behind this new robot is to design a system exclusively for the installation of the

nozzle dam. In this system the dam is fully integrated in the robot to provide a faster

installation. The bolting system is also unique and fits both the robot and the dam for a









quicker and safer operation. The robot can be used on any existing nozzle and is installed

without personnel entering the steam generator. The manipulation of the system is

greatly simplified compared to the previous design and does not require extensive

training for the operators. Foster-Miller successfully tested this robot on a mockup in

1994 and has shown significant exposure and critical path time reduction.

Once the nozzle dam is installed and secured the inspection or the maintenance of

the primary side of the steam generator can begin. An eddy current probe performs the

inspection of the inside of the tubes. This probe is sent into the tube by a manipulator

robot from the bottom of the cold or hot leg of the tube bundle. The deterioration of the

tubes or their leaks can then be documented. The most challenging operation is to clean

the inside of the tubes. When the reactor is working, deposits accumulate inside the

tubes, increasing the flow resistance through the steam generator and reducing the total

efficiency of the process. The chemical cleaning method consists of injecting a chemical

solution to attack and dissolve the deposits. This technique generates a large volume of

mixed waste that is undesirable. The SivaBlast mechanical cleaning system from

Siemens is a unique solution that is both clean and efficient [25]. The principle is simple:

steel beads are blasted from the cold leg of the primary side and are collected at the hot

leg with the waste. A generator provides the pressure to a blasting nozzle, a vacuum tool

receives the beads and a reclaimer separates the beads from the debris. This vacuum tool

avoids any contamination such as dust and small wastes. The beads are then washed and

reused. This method has already been used on non-nuclear applications. The first

challenge is operating remotely on each of the tubes, the second challenge is to clean all

3550 tubes contained in a single steam generator on schedule. A robot with a strong









dexterity can solve the first challenge. The goal is to access all tubes of the cold leg and

their corresponding end on the hot leg. Initially, the Telbot robot was used, then the

Flexivera manipulator system followed. A description of those two systems is given in

the next paragraph. The second challenge is to improve the speed of the cleaning

operation. The software ROBCAD is a program that optimizes the robot path planning.

This method allows a speedier cleaning operation since the robot cleans all the tubes

corresponding to its arm configuration. Finally, the design of the blasting tool was

developed to include two nozzles. The robot interface consist of a user interface to

control the manipulator, a ROBCAD based simulation system, and several monitors to

follow the operations. The role of the operator is greatly simplified by the high degree of

autonomy offered by the hardware and the software. Three camera sets provide many

views of the cleaning operation; views of the steam generator man-ways, views of the

two cold and hot sides of the steam generator and views of the vacuum and blasting tools.

This SivaBlast system has been tested in the Point Lepreau power plant in 1995: it

removed 787 kg of deposits from 8209 tubes.

A description of several mechanical arms that are mainly used for steam generator

inspection or maintenance is next described. The ROSA III robot is the evolution of the

ROSA I and II models [26]. This robot is a remotely controlled maintenance and

inspection robot. ROSA III is a mechanical arm that can be equipped with tools for

general-purpose work. It is mainly used, however, for steam generator operations

through the manhole port to reduce personnel exposure. The control station consists of a

highly graphic interface allowing maximum flexibility for the operator. ROSA III is

available from Westinghouse Electric. The Cobra system was specifically designed to









increase maintenance productivity and to reduce personnel exposure when workers

access the inside of a steam generator [27]. It is a lightweight electric manipulator that

inspects and repairs the lower tube sheet. Cobra was tested in 1992 and is deployed by

B&W Nuclear Service. The Telbot robot is a 6 degrees of freedom mechanical arm

developed for application in areas inaccessible for humans. Its main advantage is its

great flexibility; each arm can rotate 360 degrees continuously. It lacks electrical

components, having neither a cable nor a motor in its arm. All sensitive elements are

protected in the base that contains drive motors, drive gears, encoders and cabling.

During steam generator maintenance, the arm is installed through the man-way, while the

base remains outside. The control cabinet can be associated with this base or located in a

safer area. The Flexivera manipulator system was exclusively designed for steam

generator operation. It is small, lightweight and easy to install in the steam generator

bowl. The four axes of the manipulator arm are controlled by a computer system located

outside of the radiation area and linked to Flexivera by a cable. It is now one of the most

efficient robots for inspection and maintenance of the primary side of steam generators.



Inspection of Reactor Vessel


Specific regulations require reactor vessel inspections on a periodic basis. The

goal is usually to look for cracks and to probe the welds visually with an ultrasonic probe.

These operations imply several challenges. First, all the operations are accomplished

underwater since the reactor is completely flooded. Secondly, all the equipment in the

reactor has to be taken out prior to inspection. That means that the reactor is out of

service during the maintenance period. Immobilization of the reactor can cost from









$500,000 to $1,000,000 per day; therefore, the duration of the process must be minimized

in order to increase the profitability of the plant. The third challenge is the accessibility

of the welds. Early design of reactor vessels did not include inspection guidelines, nor

provide easy access to sensitive parts of the vessel. For example the inspection of the

Yankee Rowe PWR reactor vessel was challenging because of a permanent thermal

shield, which stands 2 inches inward from the vessel wall [28]. In such a case, the NRC

(Nuclear Regulatory Commission) would grant relief and waive a requirement for an

inspection of these inaccessible parts. Approval for relief and waiver is now more

difficult to obtain, especially for a license renewal.

The inspection of BWR (Boiling Water Reactor) vessels is often much more

challenging than the PWR inspections because of their highly complex internal structure.

BWR's contain hardware that stays in the vessel and occupies a great deal of space,

which makes it difficult to deliver the ultrasonic probes or the detectors at the right

location. In Tsuruga unit-1 for example, an inspection tool must pass through a gap of

only 14 cm to access some of the welds [29]. This restriction leads to some technical

difficulties that are a result of a shroud that covers the entire bottom half of the reactor

vessel, up to 12 meters below the water surface. This challenging inspection was

resolved by the use of a complex telescopic mast and arm setup. The upper part of the

vessel is probed with the upper mast in its highest position and the lower mast deployed.

The operator then moves an ultrasonic detector with a mechanical arm placed at the end

of the mast. The inspection of the middle part of the vessel is the most delicate operation

since it requires the insertion of an arm through the narrow gap between the shroud and

the vessel wall. After the insertion is accomplished, the operator moves down the upper









part of the mast. The entire setup has 9 degrees of freedom and is attached to a platform

that moves on a ring guarder around the vessel. The operator controls the system from

the platform without the help of a visualization system. About 75% of the formerly

inaccessible weld can then be accessed, probed and documented. The Tsuruga unit-1 was

successfully inspected with this tool in June 1991.

The internal parts of a PWR are completely removed during the maintenance

operations. Consequently, access to the welds is much less a problem than for a BWR.

The design of inspection robots for PWR is more focused on maintenance costs and time

reduction. These savings can be accomplished by optimizing the installation and

operation duration (critical path time), by reducing the personnel and the hardware (polar

crane) needed for the inspection. The techniques that have been used in the past are

based on the same principle: a rigid telescopic mast attached to a massive base at the top

of the vessel. The movements of the arm and the mast make it possible to inspect the

entire vessel. The major disadvantages of such a system are its weight and bulk. A

central mast manipulator (CMM) from Siemens weighs 16,000 kg and requires three

trucks for transportation. An automated reactor vessel inspection system (ARIS) from

B&W Nuclear Technology is also bulky and weighs 13,000 kg. It is obvious that the

installation of such equipment on the top of the reactor vessel creates handling problems

that cost time and money. The installation and calibration of the system often requires

more than three days [30]. The personnel needed for this inspection are numerous and

are kept from other tasks. A lighter system has been designed and used by the PAR

company: the inspection module weighs a total of 1800 kg. The major difference with

the previous technology is that a bridge is not needed at the top of the vessel. Instead of









being controlled from a platform at the top of the reactor, the mast is fixed on a tripod

structure attached to the vessel wall completely under water. This system is much

cheaper and easier to handle than the previous tools but its weight still requires the use of

the polar crane during the installation. The Mitsubishi advanced ultrasonic testing

machine (A-UT) is an innovative method that has revolutionized the reactor vessel

inspections. This robot does not have a central mast but has a small base held to the

vessel wall by four suction cups. This A-UT robot scans the vessel and then swims or

moves to its next location using its 8 wheels and 6 thrusters. Since this robot does not

have an external location reference, it uses several lasers to determine its orientation and

its position inside the vessel. The A-UT weighs only 300 kg and requires little

installation and little handling (no polar crane). The disadvantages are the complexity

and the reliability of the displacement system (28 degrees of freedom) but also the fact

that the unit has to swim to the surface to change the ultrasonic tools. The URSULA

(Ultrasonic Reactor Scanner Un-like Aris) robot that appeared in 1995 represents the

most highly developed system available today [31]. The general design is fairly close to

the Mitsubishi A-UT; however, URSULA uses three suction cups to attach itself to the

vessel wall while a six-degree of freedom arm scans the welds with an ultrasonic module.

The robot uses thrusters to swim to its location and uses lasers to keep track of its

position. All the modules of the robot are neutrally buoyant to minimize the load on the

thrusters and the vacuum pump. URSULA is so small that it can enter the reactor

through personnel hatch and two units can work together in the vessel, offering

considerable timesavings. The robot can choose from several ultrasonic tools and

calibrate them while being underwater. The ultrasonic head allows the inspection of all









types of welds: circumferential, longitudinal, bottom head, nozzle inner radius, nozzle to

vessel, nozzle to pipe and safe end to elbow. All the sensors and control equipment is

installed in the robot, therefore, only the signal and communication lines link the robot to

the operator with fiber optic cables. This operator monitors the operations through video

cameras and a graphical interface while an efficient automated system gives significant

autonomy to the robot. The time reduction for the inspection of a typical four loop

Westinghouse plant is reduced to four days by using two URSULA robots, whereas eight

to ten days was usually required.



Pipe Inspection


Reactor vessel welds are not the only welds that are under stressful conditions.

BWR and PWR reactors have pipes that also need inspection of their welds. Two types

of pipe inspection are available depending on the access possibility; welds from the

inside or the outside of the pipe. For outside inspection of the pipes the challenge, once

again, is the access. In BWR for example, jet-pump risers are located between the reactor

vessel wall and a shroud in a gap often smaller than 30 cm. The concerns with such

pump risers are the welds at and near both ends of the riser elbow. A robot has been

specially designed to access and inspect these welds. This robot is controlled from the

top of the reactor and is lowered into position between the core shroud and the reactor

vessel with a long-handle pole. The robot then uses a pneumatic clamp that attaches the

unit to the pipe. The inspection is accomplished with an effector that moves the

ultrasonic sensors to scan the pipe around the pipe and the elbow. Pipe crawlers are

widely used for inside inspection of pipes. These small robots use tracks, wheels or









thrusters to navigate deep in the pipe. Their task is often limited to a visual inspection of

the pipe with a CCD camera [32] but a few robots are equipped with ultrasonic

transducers or radiation detectors [33]. The challenge for these robots is to navigate deep

into the pipe, through turns and elbows. The shape of the pipe crawler robot adapts to the

pipe. These robots are often flexible and can be as small as 2 inches of diameter. The

connection with the operator is realized with a highly flexible wire that carries the power,

controls and the signals. For difficult access like with a reactor vessel nozzle, the pipe

crawler can be delivered in the pipe by another robot. The nuclear industry is one of the

many users of pipe crawlers. A wide variety of sizes and models are now available as

well as required equipment. The operation of pipe crawlers presents little risks. Many

years of experience have improved their reliability.



Underwater Inspection


Water provides sufficient shielding against radiation at a very low cost. Many

nuclear power plants operations are conducted under water for these reasons. Many

robots are therefore designed to operate or navigate under water. Previous technology

used long poles or telescopic arms to perform visual inspection and to retrieve small

object [34]. This method is not only costly, but also requires substantial time and some

personnel exposure [35]. Submersibles provide a great deal of help to reduce these costs.

First, submersibles can provide underwater views of an operation that would be very

helpful; to a polar crane operator for example. Second, submersibles require little

maintenance and mission preparation. Third, they are very flexible, easy to use and fast;

which is very useful in emergency conditions. Finally, submersibles are affordable, easy









to upgrade and offer little risk of damaging reactor equipment. However, there are few

shortcomings; submersibles are often limited to visual inspection and their size cannot

allow them to go in very tight spaces. Their payload is small so they cannot handle a

large load of onboard equipment. The radiation hardening strategy of submersibles is

simple, all of the electronics except for the CCD camera is removed from the robot. The

control system is located in the operator's desk and is linked to the robot with cables.

This method limits the risk of failure; moreover, all the submersible parts are secured and

linked to the surface so that no foreign object can be lost by accident. For most

applications, only two workers are needed to operate a submersible. A cable tender is

located at the top of the pool and his task is to submerge the robot into the water and

make sure that the umbilical cable does not get tangled. The operator is located far away

from the robot, to insure a low-radiation field. Minimal training is required for the

operator and the controls of the robots are simple. The more one displaces the joystick,

the more thrust is applied. Several monitors provide the operator with all necessary

views and parameters. The in-vessel visual inspection of reactor elements is the primary

application of submersibles. Their maneuverability and small size is particularly useful

in BWR where the access is limited by the internal hardware. Since submersibles are

used in many areas like the petroleum industry, ocean exploration, or the military, many

models of submersibles are available from several companies around the world. One of

the first submersibles used in the nuclear industry was the MiniRover [36]. It was used in

the fall 1987, in Salem-1 for underwater reactor core inspection [7]. Deep Ocean

Engineering Inc. manufactures three types of robots for the nuclear industry [37]. The

Firefly robot is a small sized (190x153x356 mm) submersible that can inspect confined









spaces, such as beneath the BWR core plate. It is equipped with a CCD camera. The

Dragonfly robot is a version that is larger than the Firefly. It can carry additional

equipment such as cameras or lasers. Both the Firefly and the Dragonfly have a body

geometry that prevents being affected by thermal water currents. The Phantom 500 robot

is the biggest submersible made by Deep Ocean Engineering Inc. It is capable of visual

inspection but can also retrieve small objects in a reactor or perform simple ultrasonic

testing. The size of the robot is a critical factor for BWR inspection. Toshiba has

developed a very small (15 cm diameter, 20 cm long) ROV (remotely operated vehicle)

that navigates underwater to inspect BWR core internals [38]. Four thrusters assure the

propulsion of this ROV that is linked to the operator via a small flexible and buoyant

cable. This robot is limited to visual inspection because it only has a CCD TV camera.

The recovery of such robot after a problem in an inaccessible part of the reactor is a risky

operation, potentially dangerous for the reactor. Therefore a support system has been

developed to deliver the ROV in the bottom region of the reactor vessel. The robot is

held at the end of a long pipe, similar to a control rod guide tube. In addition of holding

and releasing the ROV, this support system is equipped with two cameras, a light, and the

responsibility of managing the cable unwinding mechanism. This technique has many

advantages: it allows a much easier recovery of the ROV in case of problems, it avoids

interference between the cable and the ROV cable and finally the additional cameras

provide an extensive view of the work. From the radiation hardening point of view, the

only radiation sensitive element of this system is the CCD camera and this has been

certified up to 200 Gy. This equipment has been successfully tested in actual reactors

and has demonstrated excellent timesavings.












Handling and Processing of Waste


Manipulators are essential in a nuclear environment. They are used in many other

disciplines to process repetitive and delicate work. Their mission in the nuclear industry

is different: they replace the human arm where the radiation level compromises the safety

of the personnel. For a long time the manipulation of hazardous material has been

executed by a master/slave system. The operator manipulates a master arm that is

mechanically connected to a slave robot in a hot cell. The robot reproduces every

movement of the operator. The advantages of this system are its simplicity and

affordability. The shortcomings are a low payload and a limited distance between the

operator and the robot. Teleoperated systems have been developed to avoid such

problems. In such systems, the symmetry of movement is not reproduced mechanically

but electrically. Sensors are measuring the displacement of a master arm, then the

information is sent to actuators on the slave robot. Since the cost of custom-built robot is

often excessive, the strategy is to use commercially available robots. Unfortunately, the

use of robots in other industries is different; the manipulators are used to perform a

preprogrammed task over and over, and are not controlled in real time by operators.

Some modifications are thus needed. For radioactive environments, the requirements for

a teleoperated robots are a good sealing of the parts to avoid contamination, the

installation of a force feedback system, an acceptable level of radiation hardness, a good

payload/mass ratio, a high reliability and modularity for easier maintenance and an easy

integration into embedded equipment. The installation of a radiation-hardened force

feedback system is very important when upgrading a commercial manipulator to its









nuclear equivalent. Since the manipulator is used in place of a human arm, it is not

sufficient to simply visualize an operation to be able to operate correctly. A good

example is the handling of an egg. An operator can see how well a gripper holds an egg

on a monitor but this is not enough. This operator needs a force feedback system to avoid

squeezing the egg too hard and avoid breaking or dropping it. This feedback is made

with force and torque sensors and requires a significant amount of electronic and controls

algorithms. A commercial manipulator has many advantages including low cost, high

payload, speed and good reliability. The NEATER 760 (Nuclear Engineered Advanced

TeleRobot) developed by AEA Technology is a modification of the commercial Puma

762 robot [39-40]. The on-board electronics of the Puma were redesigned to a tolerance

of 106 Gy of total dose. A modular design also allows a quick replacement of a failing

unit. The maximum load of the NEATER robot is 20 kg at a maximum reach of 1.4 m.

The same strategy has been used by the French CEA for the modification of the

STRAUBLI RX90 robot [41]. The CEA has also developed a computer based

teleoperation control system named TAO2000. This program integrates the geometry of

the robot and the non-linearity of its sensors to provide a virtual model of the robot and

its trajectory. An automated path planning and an efficient man-machine interface can

then be created and improve the quality of the teleoperation. The TAO2000 system is

also used with the BD250 dexterous arm also developed by the CEA. Unlike the

modified RX90, the BD250 is not a modification of a commercial robot [42]. It was

designed to meet unique requirements in the nuclear industry. The BD250 is a 7 degrees

of freedom mechanical arm. A succession of roll and pitch axes provides a very high

dexterity to this robot. The payload is 25 kg, while the mass of the robot is 75 kg; the









payload to mass ratio is very good. The total length of the robot is 1.4 m and the arm can

be introduced into 25 cm diameter fitting. A master arm located with the operator in a

safe area controls the BD250. The on-board electronics and the force feedback sensors

are radiation resistant up to 10 kGy. Two BD250's can be associated to form a high-

performance mobile working platform. This dual arm system is suspended by a mobile

crane and is much more mobile, lightweight and smaller than any other dual manipulator

robot.




Decontamination and Decommissioning



Surface Cleanup


Despite the constant care and maintenance of the equipment in a nuclear facility,

it may happen that radioactive contaminants are spilled and must be removed. More

often some facilities and hardware need to be decontaminated after being in contact with

radioactive contaminants during operation. In most cases the spill is small enough that

the workers are protected by mobile shielding equipment and can clean up the spill.

However, the need to reduce personnel exposure and the ALARA (As Low As

Reasonably Achievable) concept are pushing the use of robotics for cleaning and

decontamination work. Conventional, general-purpose robot or custom-made actuators

as well as cleaning robots are used depending on the cleaning needs and the contaminated

facility geometry. Several examples are given below.

A master slave manipulator has been specially designed to cleanup a flooded

radioactive waste area at Nine Mile Point 1. The cleaning operation was required to









remove all contaminated materials including the barrels and to decontaminate the area to

acceptable levels. A robot called TROD (Tethered Remote Operating Device) consisting

of a remote manipulator Gamma 7F with 6 degrees of freedom and 2 meters reach was

specially manufactured by RedZone robotics [43]. This mechanical arm is mounted on a

moving base that navigates in the contaminated area by an existing conveyor system

fixed to the ceiling of the room. This method of navigation provided greater access to the

waste, and better maneuverability than a ground based vehicle. The Gamma 7F master

slave manipulator is hydraulically powered and controlled by an operator, its radiation

resistance is greater than 105 Gy. The slave arm replicates the movements of the six

degrees of freedom master arm. Even thought no force feedback is provided, the master

arm was designed to increase the operator comfort and efficiency. The TROD system

was deployed for 7 months in 1990. It accomplished many tasks including removing

debris and barrels as well as decontaminating walls and floor. The use of TROD was

efficient and saved between 1180 and 1960 man mSv of personnel exposure. Cleaning

operations rarely require custom-made robots, like those in the previous example. In

most cases commercial robots are adequate, sometimes with a few modifications. That is

the case with the cleanup of radioactive tank nozzles by the ANDROS Mark VI robot at

the Susquehanna nuclear power plant [44]. In that case, the robot was introduced into a

waste-processing tank containing radioactive resin, where its goal was to unplug a flow-

mixing nozzle with high-pressure water. Remotec manufactures the ANDROS robot as

well as other general-purpose robots. This mobile platform includes 6 tracks, a

manipulator arm with a 16 kg maximum load and two video cameras. The unique design

of the tracks allows the ANDROS to move on very uneven ground. The control









electronics are installed on board, sealed in the main body. No radiation hardening is

required since the dose rate in the tank was as low as 0.03-0.05 Gy/h. The operator

controls the ANDROS via a long and flexible cable. This cable includes the control lines

and video signals but no power lines since the robot carries its own batteries. ANDROS

was modified for this application by the addition of a high-pressure nozzle on its

mechanical arm. The robot was lowered into the tank through a man-way where an

additional camera was added to provide a general view of the tank internals. Once the

nozzles was unplugged, a test was performed to check them. The robot were then

removed and decontaminated. This application of ANDROS was successful and reduces

the personnel exposure from 30 mSv to 4 mSv. This application resulted in money

savings, plus the fact that the ANDROS can now be used for other applications in the

plant. In the next example, two robots; Scarab IIA and Scavenger, were used together for

a cleaning operation. In the Waterford 3 plant, about 1 cubic meter of spent resin tank

was spilled in a pump room [45]. A remote intervention was needed because of the high

level of radiation on the floor; up to 1 Sv/h. It was impossible to simply collect the waste

because of the many obstacles located in the room. A two-robot strategy was applied.

First the Scarab 2A gathered the waste at the lowest point of the room. Secondly the

Scarab robot used a water nozzle spray to slurry the waste while the Scavenger robot

vacuumed the floor. The waste was then evacuated to a shielded container in a room next

door. The operation was a success.

Cleaning operations are far more complex after a nuclear accident. In Chernobyl

for example, the damages are so large and the radiation level so high that

decontamination of the site is not planned in the near future. After the Three Mile Island









loss of coolant accident in 1979, fuel debris remained in the reactor. Most of the fuel

contained in the reactor was removed in the eighties, but about 20 tons of fuel remained

in a unreachable zone. The access of this debris area was very challenging since the fuel

rubble was located at the bottom of the reactor vessel, beneath the lower core support

structure. An innovative system debris called the airlift system (ALS) [46] was designed

to remove the debris. A long tube ending in a articulating nozzle was installed in the

vessel. The lower end of the tube including the nozzle was introduced in 171.5 mm

access hole to access the debris area. When air is injected into the nozzle, the fuel rubble

is displaced by the flow of air and water. A lift tube sucks out this mixture. If the

upward water velocity is higher than the falling velocity of the fuel debris in the tube then

the rubble is transported upward. The separation of the fuel and water is accomplished in

a separation chamber located above the nozzle, in the reactor vessel. The fuel is then

collected in a bucket that is regularly lifted and emptied. The ALS system was tested on

a full size mock-up at the Idaho National Engineering Laboratory. The results were very

encouraging and better than anticipated.



Tank Cleanup


The DOE produced several dozen millions of gallons of liquid mixed waste

during the cold war. This waste is both highly toxic and highly radioactive. It came from

the production of nuclear weapons, with the chemical processing used in separation

facilities. At that time, the storage of the waste was not addressed because of the weak

environmental regulations, a shortage of money and the immediate priority of the arms

race. Single shell concrete tanks containing up to 4160 m3 of hazardous waste were built









to address the storage of the waste for a short period of time. Such tanks are located at

the production sites owned by the DOE (e.g., Hanford Site, Oak Ridge Reservation,

Savannah River Site, Idaho National Engineering and Environmental Laboratory

(INEEL), Fernald Site, etc). These tanks have been in use much longer than originally

planned and many of these tanks are now leaking and are creating a major threat to the

environment. Such tanks do not meet today's requirement for toxic and radioactive

storage. The waste contained in the single shell tanks has to be removed and stored in

more appropriate storage. The removal of the waste is challenging for many reasons.

First, the waste is extremely toxic, corrosive and radioactive, and is sometimes

inflammable as well as explosive. Second the tanks access is limited to only tank risers

as small as 18 inches in diameter. Third, the inside of the tank is sometimes filled with

pipes and equipment that limits the maneuverability inside the tank. This equipment

consists of cooling pipes, thermocouples and other monitoring devices. Fourth, a

requirement is that little or no new elements should be added to the waste volume. Fifth,

a decontamination procedure has to be created to avoid the spread of contaminants to the

environments. Finally, the consistency of the waste is very complex. Tanks contain

liquid, soft sludge and hard sludge. The composition of each tank is different; sometimes

a crust is formed on the top layer of waste. Trapped gases can sometimes move a

significant amount of waste. A sudden eruption of gas can damage the internal

equipment of the tank [47]. The removal of waste from in-tank storage is a long-term

process that starts with an evaluation of the tank and the definition of the waste. The

fluid part of the waste is then pumped outside the tank. Other methods are used to

remove the various forms of sludge.









The characterization of the tank containment is important to find leaks and to

evaluate the aging condition of the tank walls. The chemical composition of the waste,

its radiation activity and temperature must also be determined, as well as the sludge

consistency and depth. The DOE has developed remote tools that perform visual

inspection, ultrasonic testing and sampling. The easiest way to monitor the inside of a

tank is to lower a video camera through the 127 mm diameter of one of the tank risers.

This method has been used in cooled waste tanks at the Savannah River site [48]. In that

case, the camera was used to monitor the new in-tank precipitation process that added

high temperature and risks of explosion to the list of potential hazards. For reliability

reasons the system was kept very simple. The camera housing consists of a camera in a

vertical position and is directed to a mirror above it. Two powerful halogen lights are

installed under the mirror. The bottom part containing the mirror and the lights can rotate

providing a circular view of the tank. The mirror can also rotate on a horizontal axis and

is constantly heated to prevent fogging. The camera can be radiation-hardened up to 105

Gy or not, depending on the environment. The controls of the camera system consist of a

video monitor and switches for camera positioning. The camera housing can be lowered

in position by two systems. A telescopic metallic mast is often used but is too heavy in

many applications. The other system utilizes a motorized cable reel, which is

lightweight, compact and simple. The electrical cables feeding the video and the other

equipment are wrapped around the cable reel. Both systems have been tested on mock-

ups in 1992 and were then used in the field. The DOE Savannah River site has ever more

demanding requirements for tank inspection since the tank risers have a diameter of only

51 or 76 mm [49]. There are no appropriate commercial inspection tools for such small









diameters. A few video cameras are available but the lighting is too weak for tank

inspection. The available sampling tools are not useful since they present risks of

contamination. Specialized equipment has therefore been designed for this unique

application. When introduced in the tank riser by a pole, the camera is maintained in a

horizontal position by an electromagnet. Once in the tank, the electromagnet is released

and the camera is secured horizontally by a spring while the lighting system rotates out of

the camera housing. Two visual inspection systems have been built; one for the 51 mm

inspection and the other one for the 76 mm inspection. Several sampling cups have also

been built with the appropriate bag-out and containment modules to prevent

contamination. The remote tank inspection system (RTI) is a five-degree of freedom

manipulator attached to a telescopic mast, which is lowered vertically from a 12 inch

tank-riser [50]. The vertical mast can go as deep as 12.5 m below the ground level and

the arm has a reach of 1.8 m. Several end effectors can be attached to the manipulator to

accomplish a wide variety of missions [51]. The visual inspection end effector consists

of two cameras, two lights and a light positioning system. In operation, the first camera

provides a general colored view of the environment and the second gives a high-

resolution close-up view of details in black and white. The ultrasonic end-effector is

used to detect corrosion or other defects in the tank walls or internal equipment. The

sampler end effector collects about 120 millimeter of waste at a determined depth.

Several samples can then assess the different layers of sludge. The characterization of

each sample will give the chemical composition, the activity, the density, the consistency

and the viscosity. Finally, the last end effector is a laser that gives a 3D view of the

inside of the tank to determine the location of foreign objects. The setup using the RTI









(Remote Tank Inspection) system was tested in 1991 in a mock-up facility and was the

basis of a light-duty utility arm (LDUA) that was developed in the first half of the

nineties at the DOE Hanford site [52-53]. One of the improvements brought by the

LDUA was a bag out system that prevented the spread of contamination in the

environment during and after the removal of the robot from the tank. This work led to

the design of the modified light-duty utility arm (MLDUA) in the second half of the

nineties. Only the MLDUA has been used in an actual tank. The MLDUA description is

continued on page 40.

Once the characterization of a tank and its sludge is realized, the removal of the

waste must be accomplished. For the majority of the tanks, the removal strategy is as

follows. First, the most fluid part of the waste is pumped out of the tank. The remaining

wastes left are the soft and hard sludges that cover about one foot of the tank's bottom.

Second, a sluicing tool dislodges and mixes the sludge, then evacuates it. This sluicing

tool can be autonomous or be manipulated by the MLDUA or a mobile robot from inside

the tank. This mobile robot can also move the waste inside the tank to provide more

efficient pumping. Third, when the tank is empty the walls are scarified to remove the

thin contaminated layer of concrete that was in contact with the waste. There are no

major challenges with the transfer of liquids from one tank to another. Available

technology can be used without modifications. Plugging of the transfer mechanism must

be avoided however. A PulsAir system has been used in a few of the Oak Ridge tanks to

separate the waste components by particle weight [54]. The principle is to inject air

bubbles into the waste through large plates at the bottom of the tank. This makes the

lightweight particles available at the surface for safe transfer. After the liquids are









removed, the sludge remains. A method, widely used now, uses pressurized water to

dislodge and mix the sludge that is then collected and pumped out of the tank. The

borehole-miner extendible nozzle sluicing system operates at a pressure up to 20.7 MPa

[55]. It consists of a telescopic mast lowered through a tank riser that supports a

maneuverable high-pressure nozzle. The nozzle can be extended up to 3 m from the mast

centerline. This hydraulic motor driven chain and rod system allows the nozzle to get

closer to the sludge or the walls. The entire mast can rotate on its axis to cover the

totality of the tank. The nozzle sprays clean water or recycled slurry in order to minimize

the volume of added waste. A jet pump integrated or separated from the mast then

collects the dislodged sludge. A remote control monitoring system has been installed to

assist the operator at his console. The information given by the encoders are also

converted into an intuitive graphical view of the inside of the tank. The borehole miner

extendible nozzle sluicing system was successfully used in the summer of 1998. Five

tanks were cleaned up in less than 3 weeks with less than 20% of waste dilution. A

Scarab III robot then inspected the tank [56]. The Scarab III robot is an evolution of the

Scarab II robot previously discussed in this document. The aluminum parts were

exchanged for stainless steel elements for a greater resistance to the corrosive sludge of

the tank. The cross section was also reduced to pass through the 18 inch tank riser.

Finally, metallic wheels replaced the tracks. These modifications resulted in an increase

in weight, which required more powerful motors. The role of the Scarab robot is to take

samples and to inspect the inside of the tank. To accomplish its mission the robot has a 2

degrees of freedom gripper that can handle small tools and objects. Three cameras are

installed on the unit. One monitors the back of the robot, particularly the tether. Another









one is placed in the front to control the operation of the arm. These two cameras are

black and white, the last one is more sophisticated and provides a general view of the

robot's environment. This inspection camera is mounted on a telescopic mast at the

central part of the robot. When the inspection of unreachable tank walls is required, the

telescopic mast is deployed up to 83 inches and provides the appropriate views. The

supporting equipment outside the tank consists of an operator console and the

deployment and containment module (DCM). The control desk consists of monitors,

switches, joysticks and a VCR in a relatively compact and easily portable station. The

DCM is used to lower the robot into the tank and assures the maintenance, repair and

decontamination. To realize this mission, the DCM unit is built around a glove box.

This containment cell includes the lifting mechanism, washers, sprays and repair tools

and is designed to minimize the deployment time. The SCARAB III unit provides an

easy, versatile and cost effective tool for tank inspection. Although it is very successful

for this mission, it is also limited by its low performance for inspection and sampling.

During a performance test, several sluicing tools were installed on the Scarab III and

revealed a poor efficiency in sludge removal. In that particular case, the Houdini robot,

(see page 40), is a more efficient choice.

The confined sluicing end effector (CSEE) element is an alternative to the

borehole miner. The CSEE is used to dislodge and vacuum the sludge. In the CSEE,

nozzles are located at the end of three blades that are rotating. The nozzles eject

pressurized liquid or air in the direction of the inlet of a vacuuming tube. This tube is a

part of the hose management arm (HMA) that consists of a vertical mast connected to a

8.53 m long two section boom. Once lowered into the tank through one tank riser, the









HMA provides pressurized fluid to the CSEE and pumps out the dislodged sludge to the

outside of the tank. Since the HMA is flexible but does not have an active mobile part, a

manipulator must position the CSEE in the tank. Two actuators are used; the MLDUA

and the Houdini robot. The MLDUA is the modified light duty utility arm. It is an 8

degrees of freedom hydraulic arm [57]. Its reach in the tank has a radius of 5.03 m and

has a maximum payload of 90.72 kg. The challenge of introducing such a massive

mechanical arm into the tank through the small tank riser has been accomplished by using

a massive vertical positioning mast (VPM). Not only does the VPM manage the

installation and removal of the MLDUA but it also prevents any contaminant from

polluting the environment. A hydraulic portable unit (HPU) provides the MLDUA with

hydraulic fluids. The HPU houses the hydraulic oil pumps, reservoir, filters, cooler and

also all the electronics that control the MLDUA. The operator of the MLDUA is located

in a trailer that provides video camera displays of the cleaning operation, joystick

controls and computer assistance. Like many manipulators, the MLDUA can be

programmed to scan an area of the tank. Its repeatability is better than 0.7 cm. The

MDLUA is efficient and reliable but has the inconvenience of being bulky, heavy and

costly. The Houdini system offers greater access to the internal part of the tank and an

improved efficiency with lightweight and versatile equipment. The Houdini system is a

mobile robot that is inserted into the tank in a folded position through tank risers as small

as 61 cm [58]. Once on the tank floor, Houdini is automatically deployed. It consists of

a mobile platform moved by two parallel tracks. A collapsible plow blade in front of the

robot can move sludge in the tank to the CSEE effector. A Schilling TITAN manipulator

is fixed on the platform. This mechanical arm can retrieve objects in the tank or









manipulate the CSEE end effector. Two cameras and their lighting provide views of the

operations. A tether linked to the outside of the tank feeds the robot with hydraulic oil,

power lines and controls. This tether is covered partially in Kevlar and offers all the

strength needed for the installation and recovery of the robot. Outside of the tank, the

tether management and deployment system (TMADS) is located right above the tank

riser [59]. The TMDAS is used to lower and remove the robot in the tank. It is equipped

with a decontamination system and provides gloved access to the robot for repairs. The

TMDAS also manages the tether and is the interface between the tether, the hydraulic

lines, power lines and signal lines. The on-board electronics of Houdini is kept to a

minimum to avoid radiation damage and the control elements are located in the power

distribution and control unit (PDCU) instead. The PDCU manages all the electric lines of

Houdini. It provides the robot with the appropriate voltage and is the interface between

the robot and the operator console. The operator sits in a control trailer where critical

parameters like hydraulic pressure and filter condition are displayed. The internal views

of the tank are monitored while the operator controls the actuators of the robot with a

joystick and switches. The Houdini system is currently in its second version. The first

version called Houdini 1 was delivered to Oak Ridge National Laboratory in September

1996, where it underwent testing in a cold mock-up facility. The first "hot" application

occurred in June 1997. Several improvements aimed to increase the reliability of the

system led to the second version of the system called Houdini 2 [60]. The Houdini has

shown an impressive efficiency especially when associated with the MDLUA. The

CSEE is not the only end effector that is used by both the MLDUA and the Houdini. The

characterization end effector (CEE) takes samples of the tank wall to later determine its









radioactivity and physical degradation. The purpose of the gunite scarifying end effector

(GSEE) is to remove the thin layer of material on the wall that was contaminated by the

liquid waste. When a lattice of pipes covers the inside tank, the limited access does not

allow any use of robotics inside the tank. Five C-tanks at the Oak Ridge site have been

cleaned up with a unique technique. In this technique, the retrieval equipment is

completely external to the tank and consists of pumps and two charge vessels [61]. The

technique carries out a two-step process to remove the waste and the sludge. In the first

part of the process, the liquid waste is pumped into the charge vessels. The second part,

the same volume of liquid is re-injected into the tank through two high-pressure nozzles.

The movement of liquid dislodges the sludge and mixes the waste. If this process is

cycled many times, no sludge remains attached to the bottom of the tank and after the last

cycle, all the waste is simply pumped out of the tank. The cleaning of the C-tank W-21,

W-22 and W-23 by this method was successfully carried out. Up to 95% of the waste

was removed by this pulse-jet mixing technique. The remaining sludge is then cleaned

by a more aggressive technique. This method requires little maintenance since almost no

moving parts are involved. The hardware however, is bulky and heavy and needs

extensive decontamination. Another system, the Tarzan locomotor, avoids these

drawbacks. Tarzan is a mobile platform designed to deliver a Schilling manipulator into

a full tank containing a dense array of vertical pipes [62]. The principle of motion is

simple; two grippers at each ends of the robot help Tarzan to grasp the pipes. Once

attached to a column, Tarzan deploys itself and grasps the next one. At this point, it

releases the previous gripper and looks for the next supporting pipe. When the working

location in the tank is reached, the two grippers are secured in position and the









manipulator arm can operate. Tarzan is hydraulically powered; it includes three high

torque actuators for in-plan motion and one vertical translator. Three cameras provide

views of the operations. Each gripper requires two different views: an overview of the

room to locate the next pipe and a close-up view for an accurate grasp of the pipe. A

single camera and a mirror offers both views simultaneously and a monitor displays these

views. Each of the two grippers is equipped with this system. An additional camera

monitors the manipulator arm operation. The robot is linked to the operator and to the

hydraulic unit by a flexible tether. The cameras use a black and white image tube and

can operate without failure at a dose rate of 103 Gy/h and up to 106 Gy of total dose. The

entire robot is designed to tolerate a total dose of 105 Gy in the very harsh environments

of the tank. The first application of Tarzan is planned for the tanks of the West Valley

Demonstration Project in the state of New York.



Decommissioning


With many nuclear facilities reaching the end of their lifetimes, the

decommissioning effort is a new priority. The reduction of employee exposure as well as

cost reduction requires the uses of remote tools for contaminated parts [63]. Once the

core elements are removed, the decommissioning of the rest of the building can be

accomplished in a classical way. The classical decommissioning strategy is to dismantle

large structures from top to bottom, then transport the components to a disassembly line

where they are then cut into smaller pieces and packaged. For example the dismantling

of the Niederaichbach power plant in Germany followed the following steps: dismantling

of upper neutron shield, removal of pressure tube units, dismantling of lower neutron









shield, dismantling of moderator vessel and dismantling of thermal shield [64]. The

Idaho National Laboratory has developed a computer based planning system called

DDROPS (Decontamination, Decommissioning, and Remediation Optimal Planning

System) [65] that plans and optimizes the decommissioning operation. As a result, an

improved robotic pathway is generated and personnel exposure is reduced which leads to

savings in time and money. The remote dismantling is a very difficult operation for

remote operators. These operators need to maneuver delicate tools accurately by using a

video screen or sometimes though a thick leaded glass window. They do not have the

direct "feeling" of the tool like any other worker. In those conditions, even unscrewing a

bolt requires all the dexterity and concentration of the operator. The decommissioning of

a facility is a long working operation. The remote equipment is constantly running on the

job for a long period of time. To avoid extra costs and excessive maintenance time, the

robot must be made of robust and easily changeable off-the-shelf parts. The cutting,

sawing, drilling of small metal parts, in the disassembly line in particular, can be

accomplished by modified off-the-shelf tools. This means that holding arms must be

added as well as video cameras and that parameters such as unit current, voltage or

hydraulic pressure are monitored. The removal of elements directly from the reactor

structure is too specific to be realized by available technology. A heavy-duty dismantling

robot must be designed to ensure efficient and reliable remote operations. During the

decommissioning of the Niederaichbach unit, the following remote tools were developed.

The main development was a rotary manipulator that could use one of 63 configurations

or tools to dismantle the inside of the reactor. The crane manipulator was designed to

remove the parts disassembled by the rotary manipulator and to transport them to the









disassembly line. The ring saw, located under the reactor vessel, cut and operated with

the rotary manipulator to dismantle the cylindrical steel moderator vessel. The ring saw

was then replaced by the band saw to remove the thermal shield that protected the

moderator vessel. The decommissioning of the CP-5 research reactor at the Argonne

National Laboratory in 1997 had a different strategy. The idea was to use the same

manipulators on different platforms. This system is called dual arm work module

(DAWM) [66-67]; it consists of two Schilling Titan II mounted on a 5 degree of freedom

articulation platform. The DAWM module can be installed on a ground mobile vehicle

called Rosie, or on a platform suspended to an overhead boom or to a crane. This last

configuration was used in the CP-5 reactor and renamed the dual arm work platform

(DAWP). In the CP-5 case, the platform was suspended by a crane and used Titan III

manipulators that were easily decontaminated. The DAWP robot undertook the complete

dismantling of the reactor. Its operation also involves the cutting of the metallic parts and

removal of the graphite blocks and lead bricks. The DAWM and DAWP platforms

include 5 on-board computers based on a UNIX development system. Several cameras

provide a good view of the manipulator operation. The electronics did not have a

radiation-hardening requirement. A bundle tether contains the power, signal and control

lines. The operator is located in a low-radiation room where monitors, graphical

interfaces and joysticks and switches assist the operation. The decommissioning of the

CP-5 reactor has been successfully completed with the DAWP system. The main area for

improvement in the next application is the user interface, which is critical for a lengthy

operation like decommissioning. A similar system is planned for the remote dismantling

of a spent fuel reprocessing facility in Karlsruhe, Germany [68]. In this building, several









hot cells are accessible from the top. A dual master/slave arm system supported by a

crane will operate in the hot cell until completion of decommissioning. This system is

called the manipulator carrier system (MCS). The mechanical arms are two

electromechanical master slave manipulators (EMSM) with 8 degrees of freedom, a load

capacity of 100 kg and a reach of 2.8 meters. Several tools are available for the EMSM:

a shear, grinder, saw, drill-machine; spray for decontamination and radiation detectors.

The unit also supports video cameras, microphones and other measuring devices. All the

major unit of the MCS and the crane are redundant to avoid a need to intervene in the

contaminated environment. This equipment has been improved during and after realistic

tests in a mock-up facility carried out from 1995 to 1997. The actual demolition of the

hot facility will be achieved in 2003. The robots that were described above are designed

for the complex job of dismantling the reactor vessel. There is a requirement for a track-

based excavator to manipulate the rubble in a more efficient manner as well. The Haz-

Trak robot can meet this requirement [69]. This vehicle is similar to a commercial

excavator; it has the required power and efficiency. A single operator remotely controls

this robot. Force feedback allows the user to "feel" the reaction of the machine when it

finds buried objects for example. The control electronics are located inside the machine

and has no radiation hardening requirements. Modular design however, allows an easy

replacement of the equipment. The robot is power independent because it uses its own

diesel motor. The communication link to the operator is either an optic fiber or a RF link.

The operator has access to several remote views of the operations and also has displays of

the robot parameters; oil pressure, temperature, etc. The control desk features the usual

tools of an operating desk: joysticks, switches, monitors, graphical user interface and









VCR. A task recall feature is installed in addition to the force feedback system. This

capability allows the operator to "teach" the robot up to 255 routines some running as

long as 10 minutes. These routines are then executed automatically, under the

supervision of the operator. The weakness of the HAZ-TRACK is its low radiation

hardness that limits its uses to the final phase of decommissioning. A German firm: Mak

System Gmbh has designed a heavy manipulator vehicle that is radiation resistant up to

10 kGy from Cs137 by use of tungsten shielding [70]. This vehicle, called SMF, uses a

mobile platform similar to a small tank. A powerful diesel engine drives the tracks over

big obstacles. A massive 6 axis hydraulic arm is mounted on the platform. This

manipulator can lift up to 250 kg and has a reach of 3 meters. The end-effector of this

arm can be a gripper as well as any required decommissioning tool. A radio-frequency

link from the robot to the control unit can be located as far as 1 km from the vehicle and

up to 10 km with additional relays. The SMF unit includes four radiation resistant

cameras. Two computers are used to manage the modules of the robot. A special

tungsten container protects the electronic system from Cs 137 and allows for a reliable

operation in a radiation environment as hazardous as 100 Gy/h. An additional back-up

computer provides duplicate protection and allows the recovery of the vehicle in case of

failure of the master computer. A high-level data exchange system offers reliable

communication with the operator. A fail-safe concept has been followed during the

design of the SMF vehicle and guarantees the reliability of the unit without the use of

radiation-hardened components. The SMF robot has been available since March 1994

from Kerntechnische Hilfsdienst Gmbh in Germany.









Post Accident Operation


When an unexpected problem occurs in a radioactive environment, it becomes a

potential hazard. A fast and versatile tool for situation assessment is needed without

putting personnel at risk. General-purpose robots like the Remotec ANDROS or Rovtech

Scarab are fitted for such jobs. Their versatility comes from their ability to navigate in

unstructured environments, their efficient vision system and their mechanical arm, which

allows for a wide range of actions [71]. The Chernobyl accident on April 26th 1986 has

stressed the need for robots for intervention purposes in cases of nuclear accidents. The

first robots that were sent to Chernobyl failed because of a lack of radiation hardness or

because their tethers became stuck in the rubble. The Russians were the first to design a

robot especially for the site of the Chernobyl accident. The Mobot-ChHV was on site as

soon as August 1986 and successfully cleaned the roof [72]. It was equipped with

electromechanical actuators, but did not have any on-board electronics. In the following

years many robots have been developed and have accomplished a wide variety of tasks at

Chernobyl [73]. Among them are a video inspection robot, a boring and drilling robot, a

dust and air cleaning robot and a few dismantling robots. The latest robot designed to

explore the inside of the Chernobyl sarcophagus is the result of US-Russian

collaboration. This robot is called Pioneer; it uses many features of the successful

Redzone Houdini vehicle [74]. Pioneer uses a fully electric, track-based platform. The

modularity of the vehicle allows easy transport of each separate module into the

Chernobyl building and final assembly close to the work location. Pioneer carries a

remote viewing system, a concrete sampling drill, a manipulator arm, a sensor package

and a plow bucket in the front of the vehicle. The original goal of this unit was to take









concrete samples from the wall and floor. The concrete sarcophagus that was built after

the accident seems to have a structural weakness due to the high radiation level.

However, radiation is not the only challenge for a vehicle in the sarcophagus; light is

almost absent and the rubble of the accident also makes the building unfriendly for

remote operation. The robot contains a viewing system with a color camera radiation-

hardened up to 10 MGy of total dose and a 1 kGy/h dose rate. Three additional cameras,

shielded, with lead provide a 3D map of the environment. These cameras and the four

150 W lights are adjustable and are controlled by the operator. The concrete sampling

system consists of a rotational motor, a linear thrust actuator, a 6 axis force-torque sensor

and a diamond cutting bit [75]. The drilling machine was designed with a priority for

reliability. The system was kept as simple as possible; the on-board electronics were

minimized and designed to operate without failure to an accumulated dose of 10 kGy and

a dose-rate of 35 Gy/h. The entire sampling drill can be mounted vertically or

horizontally to sample the floor or the wall. The drilling system is controlled remotely by

the operator. The environmental sensor package measures the temperature, humidity, and

gamma radiation level as well as thermal and epithermal neutron fields. The plow bucket

is used to clear the way or to move up to 91 kg of debris at a time. The manipulator has

the same 6 degrees of freedom arm as the ANDROS Mark V-A robot and has a

maximum payload of 45 kg and a reach of 1.68 m. Like the Houdini vehicle the control

electronics of Pioneer is not onboard but in the control station to avoid radiation damage.

However, the viewing system, the drilling tool and the mechanical arm control

electronics are installed onboard. A tungsten shielding box protects them against

radiation from Cs 137 and allows a total hardness of 1 kGy to 10 kGy total dose depending






50


on the incidence of the gamma field. The viewing system, the moving platform, the

mechanical arm and the drilling system are powered and controlled via a tether to a

power distribution and control unit (PDCU). The PDCU is separated into different

modules for easier transport. The control console provides all the tools for easy

operation: monitors, switches, joysticks and graphical user interfaces. The maximum

distance between the operator and the robot is 500 m.














CHAPTER 3
RADIATION EFFECTS




Definition and Units in Nuclear Engineering



Radioactivity


Radioactivity occurs with the natural decay of an unstable atomic species into a

more stable specie. The decay of an atom is usually associated with the emission of

particles or photons. This emission of particles or photons is called radioactivity and the

initial atom is qualified as radioactive. The decaying of a radioactive atom can yield

other radioactive atoms. This succession of decay can last several centuries and finally

ends with a stable element.



Activity


The number of disintegration of a radioactive source is measured by its activity.

A source activity of one becquerel (Bq) indicates that only one atom of the source

disintegrates per second. Several particles may be emitted per decay; however it does not

mean that only one particle is emitted per second. The becquerel is the legal unit but is

very small. Another unit of measurement is called the curie (Ci): 1 Ci = 3.7 1010 Bq.

The curie is the amount of radiation from 1 gram of radium.











Decay constant, Mean-life, Half-life


The decay rate is measured by the decay constant k; the probability of particle

decay per unit time. The decay constant is an absolute constant for each particular

radionuclide. There is no variation with other parameters such as temperature or

pressure. The mean life -c of a radionuclide is then c = k-1. The half-life T of an element

is the amount of time needed to decay to half of the initial number of a radionuclide

without adding any new element. The relation between the half-life and the decay

constant is: T = ( In 2 ) / k.



Energy


When a particle or photon is emitted after decay or any other reaction it has a

specific energy E. For photons, this energy is proportional to their frequency v: E = h v.

For material particles like electrons, this energy is kinetic energy. In nuclear engineering,

the energy of particles is measured in electron Volt (eV). The legal unit is the Joule (J),

and the relationship between the two units is 1 eV = 1.6 10-19 J.



Dosimetry


The interaction of particles and the matter they travel into causes a loss of energy.

The energy absorbed by a material per unit of mass of this material is called the dose.

One Gray (Gy) is the absorption of one joule per kg of material. A non-legal but

common unit of dose is the rad (rad), the correspondence is 1 Gy = 100 rad. A dose is









always referenced to a mass of material. Consequently, 1 Gy in water does not represent

the same effect as 1 Gy in silicon. The radiation effects on the human body depend on

the type of particle. The quality factor (Q.F.) is multiplied by the absorbed dose in Grays

to obtain the equivalent dose in sievert (Sv).

* For gamma, X-rays and beta Q.F.=1 1 Gy < 1 Sv
* For neutron and protons Q.F.=10 1 Gy > 10 Sv
* For alpha particle Q.F.=20 1 Gy > 20 Sv

Another unit commonly used for equivalent dose is the rem: 1 Sv = 100 rem.




Types of Radiation and their Interaction



Photons: Gamma and X-rays



Definition. Gamma rays and X-rays are identical: they both are photons with very

short wavelengths. The only difference is their origin; gamma rays come from a nuclear

interaction, X-rays come from electronic or charged-particle collision. They interact

identically: they lightly ionize and penetrate deeply into the matter; however, they do not

create any activity after interaction as long as their energy is less than 10 MeV. The

number of photons exponentially decreases with the target thickness. When a collimated

beam of mono-energetic photons enter perpendicularly into a target with an intensity lo,

the intensity of uncollided photons at depth x of the target is I (x) = Io e-'x. The

coefficient [t is the attenuation coefficient and depends on the photon energy and on the

target, especially the electron density in the target. The only protection against photons is

a thick sheet of material with a large charge atomic number Z, like lead. A way of









comparing the absorption of photons in different material is to use the tenth value

thickness (T.V.T.). The T.V.T. is the thickness of material needed to attenuate by a

factor of ten a collimated beam of mono-energetic and uncollided photons. Table 9 gives

several values of TVT for different materials and photon energies.




Table 9: Photon tenth value thickness in cm for Al, Fe, Pb and concrete
Energy (MeV) Aluminum Iron Lead Concrete
0.05 21 1.6 0.25 18
0.1 50 8.1 0.37 49
0.2 70 20 2.03 77
0.5 101 35 13 111
1 139 49 29 153
1.5 170 60 39 188
2 198 69 44 218
3 241 81 48 265
4 274 88 48 306
5 300 93 47 338
Source of data: see references [1]

Three principal types of photon interaction occur: photoelectric effect, photon

scattering and pair production.



Photoelectric effect. The photoelectric effect is the absorption of the incoming

photon's energy by an outer shell electron. This electron is then ejected from the atom

with kinetic energy equal to the difference of the photon energy and the electron binding

energy. This interaction also results in emission of luminescence X-rays and Auger

electrons. The photoelectric effect is the dominant interaction for low energy photons (<

0.5 MeV).










Photon scattering. Photon scattering is by definition the scattering of an incoming

photon by an electron. This scattering can be coherent (the photon energy is conserved)

or incoherent (the photon energy is partially transferred to the electron). In both cases the

photon has its trajectory modified and the electron is ejected from the atom. The most

common scattering is Compton scattering.



Pair production. This interaction is dominant at high energy and occurs only if

the photon energy is greater than 1.022 MeV. In the electric field of a nucleus or an

electron, a photon is spontaneously annihilated and converted into a electron-positron

pair. The positron and the electron have a total kinetic energy equal to the difference of

the initial photon energy and 1.022 MeV.



Beta: Electron and Positron


A P- particle is a free electron, a P+ particle is a positron. Apositron is what is

known as antimatter; a positron has the same weight as an electron but its charge is the

exact opposite. A positron does not travel very far because it is quickly annihilated by an

electron from the material and results in two photons. When travelling into a material,

the Coulomb force of the bond electrons interacts with the P3 particles. The exchange of

energy that occurs excites the atoms or ionizes them. In that case the energy is

transferred to the ejected/excited electron. The succession of accelerations and

decelerations also generates the emission of photons called Bremsstrahlung photons. The









range of beta particles is limited; usually a simple aluminum sheet can stop them (Table

10). Beta particles do not create any new radioactivity in a material.




Table 10: Range of electron in aluminum
Energy (MeV) 0.01 0.03 0.05 0.1 0.25 0.5 1 3 5
Range ([tm) 0.06 0.6 1 5 20 60 160 550 940
Source of data: see references [1]


Heavy Charged Particle


Heavy charged particles are ions like protons (Hi ), alpha particles (He4+) or any

other ionized atom. These particles are absorbed principally by scattering from an atomic

electron and from atomic nuclei. The stopping power is the rate of energy loss of the ion

per unit length, the mean range is defined as the distance the ion travels before coming to

rest. This range is usually small and does not exceed a tenth of a millimeter for alpha

particles (Table 11) and a millimeter for protons (Table 12). Heavy charged particles do

not create any new radioactivity in a material. Heavy charged particles that meet in

nuclear interactions are stopped by the plastic package around components.

Consequently they are not a threat to electronic systems. The effects on plastic or surface

can be significant however.









Table 11: Range ([tm) of alpha particles in Al, Pb, water and air
Energy (MeV) Aluminum Lead Water Air
0.01 0.1 0.05 0.2 240
0.1 0.6 0.4 1 1330
0.5 1.8 1.5 3 3310
1 3.3 2.5 5 5520
2 6.6 4.6 11 10800
5 22 14 37 36700
Source of data: see references [1]



Table 12: Range of protons in aluminum
Energy (MeV) 0.1 0.3 0.5 1. 3 5 10
Range (um) 0.7 2.7 5.3 14.5 78 182 604
Source of data: see references [1]


Neutron


Neutrons are uncharged particles with approximately the same mass as protons.

Neutrons are classified into three categories depending on their energy; thermal neutrons

(0.02510

keV). Since Coulomb forces cannot interact, neutrons are very difficult to stop. Fast

neutrons lose their energy by elastic scattering with the atoms. The transfer of energy is

greatest when the neutron collides with a hydrogen atom. This explains why materials

containing a lot of hydrogen, like water, are the best shields against neutron. Fast

neutrons are slowed down after multiple scattering; they become thermal neutrons. The

probability of nuclear reactions in this range of energy is much higher. The number of

protons or neutrons of the target nucleus can be modified and lead to radioactivity if this

new element is unstable. A nuclear reaction also generates a number of other particles

like gamma, beta or alpha. The damage created by gamma rays on robotics and









electronic systems, are far more damaging than the damage by neutrons. In addition

neutrons are much less common than gamma rays. This explains why the effects of

neutrons are not a concern when it comes to designing a robotic or electronic device for

nuclear application. The only exceptions are the in-core and near-core applications.



Conclusion


The previous paragraph presented radiation types and their interaction with

matter. This presentation shows that gamma rays are the biggest threat to robotics and

electronic systems for terrestrial nuclear environments. Alpha and beta particles can be

stopped with a light shield and are therefore non-threatening to the reliability of a system.

This is not true outer space.

When gamma rays interact with material, they create two effects. The first effect

is ionization. Photoelectric effect, Compton scattering and pair production eject electrons

from the atoms of the material. These ejected electrons can create secondary reactions.

The result is a track of ionized atoms in the bulk of the material. The second effect is

atomic displacement. Sometimes the atom receives so much kinetic energy at the site of

interaction that it leaves its initial location in the material. This displacement creates

additional atomic movement on its track that may result in a cluster of defects into the

atomic lattice. The immediate and long-term results of ionization and atomic

displacement strongly depend on the material. The next few paragraphs will discuss

these effects.









Radiation Effects on Passive Elements


This section describes the effects of gamma rays on a wide range of materials

other than semiconductors. It is extremely difficult to define a level of failure. The

performance of a material is defined with many physical properties: mechanical,

electrical, thermal, optical, etc. A slight change in any of these properties may cause no

effect or may have tremendous effects on a system. Therefore the following values of

radiation damage threshold are only an estimation of radiation effects on materials. The

data presented in this chapter come from various origins. K.U. Vandergriff from Oak

Ridge National Lab has published an extensive set of data on radiation damages on

materials [76]. Several other sources give some interesting information on radiation

effects [1,3,77,78].



Inorganic Materials



Metals. Metals are immunized against damages from gamma rays. The metallic

structure is very resistant to radiation. Exposition of metals to very high dose rate

generates some heat that may be indirectly damaging to the system. After long-term

exposure (several decades) to a very high dose rate, some defects may be detected like an

increase in tensile and yield strength and a decrease in ductility. These defects can be

annealed, and the metal would recover its mechanical properties. Table 13 gives some

damage thresholds for commonly used metals.









Table 13: Radiation damage thresholds on metals
Metal Threshold level (Gy)
Aluminum and its alloys 5. 1011
300 series stainless steel 1. 1011
400 series stainless steel 5. 1010
Iron 3. 1010
Copper 2. 1010
Brass and bronze 1. 1010
Nickel and its alloys 1. 1010
Beryllium copper 6. 109
Source of data: see references [76]


Ceramics. Ceramics are used as a dielectric in capacitors, and also as coatings to

replace plastic coatings. Ceramics are more resistant to radiation than organic materials

but are not as tolerant as metals. The radiation effects on ceramics are a dimensional

swelling and therefore a decrease of the density. Table 14 shows some damage

thresholds for common ceramics.


Table 14: Radiation damage thresholds on ceramics
Ceramics Threshold level (Gy)
Alumina 5. 1010
Silicon carbide 6. 108
Mica 5. 107
Quartz 2. 107
Glass, flint 2.5 105
Glass, borosilicate 1. 105
"Vycor" glass 5. 104
Source of data: see references [76]

See the next paragraphs for a description of the radiation effects on specific

materials like glass, crystals and optical fibers.









Organic Materials



Polymers and plastics. Polymers are long, chain-like molecules, with a relatively

small number of chains per unit volume. Radiation produces two effects on polymers:

cross-linking and chain-scission. Cross-linking occurs when two molecules are bound

together by the radiation effects on electrons. The opposite effect also occurs: chain-

scission, which shortens molecules to smaller chains. Both of these effects occur

simultaneously in polymers. The predominance of one effect varies with polymers and

experimental conditions. Few plastics are actually irradiated to improve their mechanical

properties but in most cases radiation degrades plastics. The damages result in cracking,

blistering, embrittlement and an increased sensitivity to mechanical stress. Some

polymers are particularly sensitive to radiation. Teflon (PTFE: polytetrafluorethylene) is

degraded after only 100 Gy. The use of Teflon is prohibited in radiation environments.

Halogenated polymers and fluorocarbons release corrosive chemical like HCL and HF

when irradiated. Not only do these gases affect the polymer but also the other equipment.

Such polymer categories includes PVC, PVDF, Teflon and Viton. PVC does not suffer

important degradation under irradiation but the hydrogen chloride slowly shows its

effects and completely destroys the integrity of the plastic. Temperature influences the

rate of degradation as well as the composition of air. An increased concentration of

oxygen in the air produces more oxidation and increases the number of chain-scission.

Table 15 gives the radiation damage classifications of a few plastics. See reference [76]

for a complete set of radiation test data.









Table 15: Radiation tolerance of plastics
Radiation resistance Polymer
Glass fiber phenolics
Asbestos filled phenolics
Epoxy systems
Polyurethane
Highest radiation resistance Polystyrene
Mineral filled polyester
Mineral filled silicones
Furane-type resins
Polyvinyl carbazole
Polyethylene
Melamine-formaldehyde resins
Urea formaldehyde resins
Moderate radiation resistance formaldehyde resins
Aniline formaldehyde resins
Urfilled phenolic resins
Silicone resins
Methyl metacrylate
Unfilled polyesters
Poor radiation resistance Cellulosic
Polyamides
Teflon
Source of data: see references [76]


Coatings. Organic coatings consist of a thin polymeric film, used for esthetic

purposes and protection against corrosion. They are also used in radiation environments

to provide an easier decontamination. Like any polymer, organic coatings suffer

degradation from radiation. The damages may result in cracking, blistering,

embrittlement and/or surface flaking. The degradation on coatings varies a lot with their

composition and depends on factors such as; temperature, type and preparation of the

surface. Table 16 indicates the radiation damages on several coatings for different

surfaces.









Table 16: Radiation damages on coatings
Polymer base Surface Dose (Gy) Damages
Epoxy Steel 6.7 106 No failure
Concrete 9.4 106 No failure
Furan Steel rod 8.4 106 No failure
Concrete 9.4 106 No failure
Modified Phenolic 6
Modified Phenolic Steel rod 8.7 106 Severely embrittled
Concrete 6.7 106 No failure
Silicon alkyd Steel 6.7 106 No failure
Concrete 8.7 106 Failed blistered
Styrene Steel 8.7 106 Failed cracked
Steel (wet) 8. 105 Failed cracked
Aluminum 2.1 106 Failed blistered
Vinyl Concrete 1.1 107 Borderline failure
Aluminum 2.1 106 Failed blistered
Vinyl chloride Concrete 1.1 107 Failed blistered
Steel 8.7 106 Failed blistered
Source of data: see references [76]

Unlike any other material, coatings are threatened by alpha particles. Alpha

particles have a limited range that is comparable to the thickness of the coating film. The

entire bulk of polymers may then be damaged by the alpha particles. Alpha particles are

more ionizing than gamma rays and therefore create more damage. If coatings are

certified with gamma particles, the damages will show up earlier than expected when

used in alpha environments.



Adhesives. Adhesives are organic compounds that are used to hold two structures

together. Adhesives are under mechanical stress when loaded. Radiation damages the

chemicals in the adhesives and decreases the number of adhesive bonds. Some adhesives

are more radiation sensitive than others are. The degradation may be accelerated with

vibrations, temperature and/or a higher concentration of oxygen in the air. Table 17

illustrates the radiation damage of a few useful adhesives.









Table 17: Radiation damages on adhesives
Adhesives Radiation damage threshold (Gy)
Neoprene-nylon-phenolic 5. 105
Neoprene-phenolic 106
Epoxy, epoxy-thiokol, nitrile-phenolic 5. 106
Epoxy-phenolic, vinyl-phenolic, nylon-phenolic 107
Source of data: see references [76]


Elastomers. Elastomers are polymers used for their compression and elongation

properties. Elastomers are used for seals, o-rings, gaskets, diaphragms and insulation.

Mechanical properties like tensile strength, compression set and elongation at break are

affected by radiation. The radiation degradation depends on the polymer base and the

type and concentration of additives. Some additives like amines and phenols may also

protect elastomers from the effects of radiation. Radiation damages are unlikely to occur

on elastomers after a total dose less than 10 kGy. See reference [76] for a complete set of

radiation test data.



Lubricants. Oil and grease are used in moving mechanisms to reduce friction, but

also to cool and to prevent corrosion. Lubricants are organic materials made of natural or

synthetic oil and a minority of additives. The type and concentration of additives control

the characteristic of the lubricant: viscosity, thermal conductivity, heat capacity,

corrosiveness, chemical and temperature stability. These parameters are affected by

radiation because lubricants are radiation sensitive like any other organic compound. The

results of the chemical degradation of the organic molecules are an increase in viscosity

that may ultimately lead to a polymerization to a solid state and a destruction of the

additives that result in modified physical properties. It is found that synthetic lubricants









are more radiation resistant than natural lubricants but few exceptions exist. The more

radiation tolerant lubricant are polyphenyls, poly(phenyl ethers) and alkylaromatics, but it

is often better to operate unlubricated in very high dose rate environments. Table 18

gives a few examples of radiation damage threshold of lubricants. See reference [76] for

a complete set of radiation test data.




Table 18: Radiation effects on lubricants
Radiation damage threshold (Gy) Lubricant
104 and below No significant radiation damages
104 to 105 Aromatic phosphates, silicones, aliphatic esters
105 to 106 Diesters, aromatic esters
106 to 107 Mineral oils, aliphatic ethers
107 to 108 Alkylaromatics, poly(poly ethers), polyphenyls
108 and above No lubricant available
Source of data: see references [76]


Optical Material


Optical materials are transparent because the energy gap between the valence

band and the conducting band is greater than 3.1 eV. Light photons whose energy is less

than 3.1 eV cannot excite carriers. Therefore the material does not absorb the

corresponding light wavelengths. Radiation creates defects that have new energy levels

in the gap, allowing greater carrier excitation and light absorption. This effect is called

darkening or browning of the transparent material under radiation.



Glass window. Remote operators visualize the inside of a hot cell through a thick

leaded window that absorbs radiation. This window also attenuates visible light,









requiring a very powerful lighting source in the hot-cell. Any darkening of a window due

to radiation affects the ability of the operator and is undesirable. The addition of less

than 2.5% of cerium oxide in the glass composition prevents the effects of darkening.

Unfortunately this additive and the lead produce a yellow tint that increases with the lead

concentration and affects visibility. A hardening technique is to put several glass sheets

between a cell and the outside with increasing lead concentration and decreasing cerium

concentration. Another drawback of the cerium and lead mix is the risk of electrostatic

discharge (ESD). When exposed to very high dose rate, a leaded window may suddenly

release its accumulated static charge. This ESD produces cracks that may affect the

integrity of the window. An ESD is more likely to happen on windows with low

conductivity. Therefore the addition of conducting elements to a window composition

reduce the risk of ESD. Table 19 presents the effects of radiation on several natural and

cerium-treated glasses.




Table 19: Radiation damages on window glasses
Total Average light Light transmission after dose at
Material dose transmission various wavelengths
MGy before dose 400 nm 500 nm 600 nm 700 nm
Crown-2 10 98% 0% 3% 25% 46%
Crown-2 protected 10 98% 60% 86% 88% 89%
Dense flint-2 50 94% 0% 1% 11% 21%
Dense flint-2 protected 10 91% 45% 83% 85% 86%
Purified fused silica 50 100% 89% 89% 89% 89%
Quartz 10 99% 35% 30% 31% 56%
Styron 690 10 75% 0% 2% 28% 56%
Vycor 0.2 99% 0% 0% 0% 1%
Vycor protected 5 99% 24% 24% 36% 61%
Source of data: see references [76]










Camera lens. The radiation tolerance of hardened CCD camera range from 100

Gy to 10 kGy. Camera lenses are altered by the same darkening effect as the window

glass, but are more tolerant than the camera's electronics. The quality of the anti

darkening treatment is adapted to the expected lifetime of a camera to minimize costs and

maintenance.



Optical fiber. Today remote operations require several dozens of transmission

and signal lines between an operator and a robotic system. Due to thick cables containing

a great number of conductors, the mobility and the maneuverability of a robot is limited.

It is very useful to replace such cables with optical fibers. Optical fibers have many

advantages compared to metallic wires. A single fiber can carry as much information

that thousand of wires, it is less sensitive to perturbations and does not have any ohmic

attenuation. Unfortunately radiation induces an optical attenuation even at low dose.

Commercial optical fibers have been tested and showed degradations up to 100

dB/km/Gy. The majority of optical fibers are therefore not suitable for applications in

nuclear environments. It is found that pure-silicate optical fibers are more tolerant to

radiation. When the core of the fiber does not contain any doping, the attenuation is

greatly decreased. This improvement is particularly significant for the infrared

wavelengths where most of the transmissions occur [79]. Such radiation-hardened

optical fibers can show an attenuation of 0.1 dB/m after 1 MGy of exposure. This type of

fiber can be very useful where short length of fibers are needed like in hot cells. There

are other performance factors that also encourage the use of optical fibers. First, the

attenuation saturates with the exposure, typically after 1 MGy. Second, an almost









complete recovery occurs at room temperature after less than one week without

exposition to radiation. This is very interesting for maintenance robots that do not stay

permanently in a radiation field. Third, a pre-irradiation of the fiber greatly decreases the

fiber sensitivity to radiation. Therefore pure-silicate optical fibers are suitable for

operations in a nuclear environments like maintenance or where short cable length can be

used. The challenge is to build the corresponding radiation-hardened conversion

optoelectronics.



Electronic and Electrical Components



Vacuum tubes. Vacuum systems were used in early electrical systems. Vacuum

tubes are now rarely used in electronics because of their bulk and power requirements.

Despite these drawbacks, vacuum tubes are still the only equipment available that can

operate under an extremely high dose rate of radiation. The hardness of the vacuum

devices originates in their simplicity; they consist of metallic parts enclosed in a glass

bottle. A mechanism with light detection or amplification is much less affected by

radiation than with solid state devices. Vacuum tubes can tolerate total dose up to 1

MGy. Many radiation-hardened cameras use vacuum tubes that avoid total dose and dose

rate effects. Some darkening may alter the optical lense but the radiation tolerance of

these cameras is determined by the tolerance of the associated electronics.



Crystal. A crystal is a passive element used to control the frequency of an

oscillator. Radiation creates defects on the crystal lattice that results in frequency shifts

and degradation of the Q value. The frequency shifts are very small and are usually









negligible. The frequency shift is worst with natural quartz because it contains

impurities. In that case, the frequency change can be larger than 1 ppm after only 100 Gy

of total dose. For some applications, it may be essential to the reliability of a system to

have a very stable crystal frequency. It is best in this case to use synthetic quartz crystals.

The rate of change per unit of dose can then be as low as 1 part in 1010 per Gray at 10

kGy [80].



Resistors. The resistor manufacturer uses different technologies that produce

different resistance to radiation. Most of the resistors are very tolerant to radiation but

oxide film resistors can fail as soon as 10 Gy. It has been found that high resistance

value resistors are more sensitive than resistors with a low resistance value. The

radiation induces chemical degradation of the materials in the resistor that leads to a

decrease in the resistance. Table 20 gives several examples of resistor technologies and

their radiation hardness.




Table 20: Radiation damages thresholds on resistors
Resistors Threshold level (Gy)
Precision wire-wound ceramic bobbin 106 1010
Metal film 105 109
Precision wire-wound epoxy bobbin 104 107
Carbon film 104- 107
Other film 103 105
Composition 102 106
Oxide film 10 104
Source of data: see references [76, 78]









Capacitors. A capacitor consists of two conducting surfaces separated by a

dielectric insulator. Many capacitor technologies are used and their radiation tolerance

differs greatly. Since the conducting surfaces are usually metallic, they are not affected

by radiation. The dielectric damage, however influences the capacitance value, the

leakage current and the breakdown voltage. All these parameters are affected after

exposure of electrolyte capacitors to radiation. Electrolyte capacitors are the most

radiation sensitive capacitors and can fail as soon as 100 Gy. Organic dielectrics are also

not very tolerant to radiation and their exposure results in leakage current and dielectric

loss. Tantalum capacitors show both radiation-induced conductivity and stored charge

variation because charges are generated when radiation is absorbed. Glass and ceramic

capacitors are the most resistant to radiation but are limited to small capacitance values.

The degradation of capacitors depends on the bias during irradiation. The bias influences

the trapping of charges created by radiation that results in long term conductivity. Table

21 shows damage threshold levels for several capacitor technologies.




Table 21: Radiation damages on capacitors
Capacitors Threshold level (Gy)
Glass 105- 108
Paper 105
Mica 104- 107
Ceramic 104 108
Tantalum 103 105
Polyester 103 10
Polycarbonate 102
Electrolyte 102
Source of data: see references [76, 78]









Inductor. An inductor is a solenoid made of a metallic wire wound around a

former. The wire is very radiation resistant but the insulator around the wire is not and

may create short circuits. The deformation of the former under radiation may also

change the overall inductance of the inductor. The radiation resistance of an inductor,

therefore, depends on the hardness of the former and the insulator materials and lies

between 10 and 106 Gy.



Cables. Cables consist of one or several metallic conductors insulated by organic

compounds. The conductors are used to transmit electrical data and power and are

radiation tolerant. The insulators prevent short-circuits and protect against the external

environments but are radiation sensitive. The number of connectors and the cable

thickness depends on the application. For mobile robots the umbilical cord is vital

because it carries the power signal and control lines. In such applications, the cable

flexibility is important and the cable integrity must be intact even after contact with

corrosive chemicals and mechanical stress. Most of the commercially available cables

(excluding Teflon) would not present significant degradation up to a total dose of 1 MGy.

At higher dose the insulator become brittle, chips, or peels and becomes much more

sensitive to mechanical stress. The most radiation tolerant cables use PEEK and

polyimide that will not fail up to 70 MGy. Polyurethane rubbers are resistant up to 50

MGy and are more flexible. Inorganic insulators like ceramics, glass or mica can be used

in high dose rate environments. Flexibility is then accomplished by coiling the wire like

a telephone cord. When a cable carries a very high frequency signal, the dielectric









changes in the insulator are a large concern, but radiation-hardened RF cables are

commercially available.



Thermocouples. Thermocouples are devices that measure temperature. They

consist of two metallic wires soldered at both ends. A natural difference of potential

occurs when the two solders are at different temperature. The simplicity of this device

makes it one of the most radiation resistant instruments. Thermocouples are even used in

reactor cores. The only limitation is the resistance of the wire insulation.



Transformers. Transformers modify the AC voltage between two electrical

networks or provide isolation. Transformers consist of two coils of wires wounded on an

iron former. This element is critical since a change in the transforming ratio may damage

or disable an entire system. Radiation damages the enameling on the wires. This leads to

shorts and changes in the transforming ration. Radiation-hardened transformers using

resistant enameling is a requirement above 1 MGy. At very high dose the magnetic

properties of the transformers are also affected.



Connectors, switches and relays. Connectors, switches and relays are as sensitive

to radiation as their polymeric components are. Plastics and polymers suffer shrinking,

cracking and other degradation of their mechanical and insulation properties that result in

shorts or loss of integrity. Internal movement in relays, external mechanical actions,

stresses on switches, and connectors also accelerate the degradations. The release of

chemical compounds may also affect the quality of electrical contacts. An appropriate









selection of the organic compounds or the use of ceramics prevents degradation due to

total dose. Table 22 shows damage threshold levels for several connectors and switches.




Table 22: Radiation damages on connectors, switches and relays
Component Dose to produce 25% damage (Gy)
Connector, polystyrene 6. 107
Connector, polyethylene 9. 105
Connector, duroc ceramic 3. 106
Connector, melamine plastic 3. 106
Relay, switch base, asbestos filled phenolformald 1. 107
Relay, switch base, unfilled phenolformald 1. 105
Source of data: see references [76, 78]

The decontamination of robotic equipment is realized after operation in radiation

environments. Connectors must be designed to avoid the trapping of contaminants in

inaccessible parts. The effects of alpha particles are also a cause of concern for small

plastic parts.



Circuit boards. Insulating and conducting mechanical supports to electronic

circuits is provided by circuit boards. The conducting parts are metallic and do not suffer

from damage due to radiation. The main board is sensitive to the total dose if it is made

of polymers. The resulting mechanical degradation may cause distortion, cracking or

modification of the insulating properties. The radiation tolerance of the board is a

function of polymer type but damages are unlikely to be significant at doses lower than

100 kGy. An important exception is Teflon (PTFE) used in UHF boards that is

particularly sensitive to radiation above 100 Gy. The integrity of circuit board is assured

by using a glass fiber circuit board or any other radiation resistant material.











Mechanical and Electromechanical Components



Ball bearings. Friction is eliminated in moving parts by the use of ball bearings.

A ball bearing consists of a cage containing metallic balls between two metallic annuli.

The cage is made of plastic or metal. Metallic cages should be the only cages used in a

radiation environment. The choice of a radiation tolerant lubricant is crucial since many

lubricants lose their viscosity after less than 10 kGy of total dose. Synthetic lubricants

are more resistant to radiation than natural lubricants, however any containing Fluor

compounds are very sensitive.



Motors. Motors work under mechanical, thermal and electrical stress. They are

designed to operate under these stressful conditions, but radiation effects are rarely taken

into account. Motors are not exclusively made of metals. They also contain a wide

variety of organic compounds like lubricants, seals and elastomers. These parts are

radiation sensitive and the harsh environment may accelerate damages to a motor. The

radiation breaks the chemical links in molecules. This results in a change in the

mechanical properties. Temperature, vibrations and the mechanical stress in the motor

aggravate these effects further. A design with radiation tolerant compounds prevents

failure and extends the radiation tolerance of motors.



Magnets. Magnetic components are used in electromechanical equipment as well

as non-volatile data storage devices. Hard magnetic materials are highly resistant to

radiation. Soft magnetic materials are more sensitive and radiation can actually improve









their magnetic property [76]. Magnetic data storage devices like magnetic tapes or disks

are very tolerant to radiation. Damage to magnetic materials can be initially observed

after a total dose of 106 Gy.



Thermal insulation. Thermal insulation is realized by low-density foam made of

polymers. Radiation damages the polymers by releasing some gas that alters the thermal

conductivity slightly. The effects on insulation are negligible, but the degradation of

mechanical properties of the foam may raise concerns.



Mechanical sensors. A wide variety of mechanical sensors measure displacement,

pressure, acceleration, vibration, etc. Metallic sensors are very resistant, as long as the

insulators and the connectors are not damaged. This is the case for strain gauge and coil

solenoid sensors. Piezoelectric sensors are also radiation tolerant and operate without

failure up to 100 kGy. Many mechanical sensors also use some semiconductor elements,

particularly accelerometers, pressure gauges, etc. Since semiconductors are very

sensitive to radiation, an assessment of the hardness of these sensors is required.




Radiation Effects on Semiconductors


Radiation effects of gamma radiation on semiconductor devices are described

here. The effects of neutrons, protons, beta particles and heavy charged particles are not

studied since these effects raise little concern in robotic systems for nuclear

environments. The bibliography for this section includes the excellent books from

Holmes-Siedle and Adams "Handbook of radiation effects" [77] and "The effects of









radiation on electronic systems" from Messenger and Ash [81]. Two guides from

Benemann [82] and from Sharp and Garlick [1] also contributed to the writing of this

section.



Physical Effects on Semiconductors.



Displacement damages. Displacement damages are created by the ejection of an

atom from its original location in the lattice. A momentum transfer from a particle to the

atom creates this displacement. Gamma particles do not have any mass and therefore

cannot directly displace an atom. Gamma ray interactions, however, produce secondary

electrons. If the energy of the secondary electron is greater than the displacement energy,

then the target atom is expulsed from its initial position. The kinetic energy of the

knocked atom depends on the secondary electron energy and on the initial photon energy.

With low energy photons, it is unlikely that an atom receives enough energy to be

knocked-out. The displacement effects are therefore minor for low energy particles, but

increase with energy. High-energy particles like neutrons, protons or electrons create

much more displacement damages than gamma radiation, but their effects are not studied

here. When an atom is ejected from its position, it creates a vacancy in the lattice. The

ejected atom may recombine with a vacancy or stay in an interstitial position in the

lattice. The vacancies are mobile and combine with other vacancies or with impurities of

the semiconductor. High-energy photons give rise to clusters of defects and low-energy

photons only produce single point defects. The interstitial atoms are not as electrically

active as a complex of defects.









Defects introduce intermediate energy levels in the gap between the conducting

band and the valence band. These band-gap defects disturb the transport of electrical

charges by several reactions [83]. First, generation and recombination of electron-hole

pairs degrade the minority carrier lifetime. Second, the trapping and compensation

effects change the majority carrier density and decrease the carrier mobility [84]. The

reduction of minority carriers lifetime affects particularly minority carrier devices like

bipolar transistors and diodes. The reduction of carrier mobility affects all

semiconductors but is often a secondary problem compared to ionization damages or

minority carrier lifetime reduction.



Ionization damages. Ionization is the creation of electron-hole pairs along the

track of the gamma particle and the secondary electrons. Only a few eV are needed to

ionize an atom and no transfer of momentum is involved. The incident particle energy is

therefore less critical for ionization than for displacement effects. Electrons leave the site

of interaction more rapidly than holes due to their higher mobility. Any solid has a

temporary increase in its conductivity due to this carrier generation. After electron-hole

generations, electrons and holes travel in the bulk under the influence of the local electric

field. The mobility of electrons is much higher than the mobility of holes, but both

charge carriers may get into defects of the lattice called traps. Charge carriers

accumulate around traps and create a local charge build-up. These traps can be single

point defects or a mismatch of interface surfaces. This accumulation of charges is

dramatic in insulators that are used to induce an electric field in the device. In

semiconductors, the trapped charges can be excited back in the conduction band because









of the narrow band gap. In an insulator like SiO2, the band gap is much larger and the

trap energy levels are above the conduction band. It is unlikely that an electron will

recombine with a trapped hole in an insulator in a short period. An important exception

is the electron tunneling effect that annihilates trapped holes near Si-SiO2 interface. If

charges are accumulated in the insulator, it changes the electric field and the

characteristics of the device. The most common insulator used in semiconductor devices

is SiO2. A controlled growth of SiO2 is processed during the manufacturing of a device.

The bulk of SiO2 has a low concentration of defects, the Si-SiO2 interfaces, however,

contains many defects due to mismatches in the bonds between the two planes. These

two types of defects produces two categories of charge trapping; Qot is the total charge

that results from oxide trapped charges, Qitis the total charge that results from interface

trapped charges. The sign of Qit T can be negative or positive depending on the Fermi

level at the interface. The trapped charges due to interface defects are located less than

20 nm from Si-SiO2 interfaces (see Figure 2 from source [77]). The bulk of silicon

provides electrons that annihilate the closest holes by the tunneling effect. The buildup

of charges and interface effects is particularly dramatic for MOS (Metal Oxide

Semiconductor) and CCDs (Charge Coupled Devices) but is secondary for bipolar

devices.










Distance from Si-SiO2 interface (Log scale)


Bulk Si-SiO2 with few deep traps {

20 nm


Bulk Si-SiO2 with many deep traps 5 nm


2 nm ...... ... ............................................. T rapp in g
Hole annihilation and
Near interface SiO 2 electron tunneling

0.2 nm ----- Slow exchange

Near interface SiOx Fast exchange

r Si-SiO2 interface
Near interface Si


Bulk Si


Figure 2: Trapping zones at Si-SiO2 interface




Annealing of defects. The damages caused by atomic displacement and

ionization can be recovered partially or even completely by a phenomenon called

annealing of the device. The physics of annealing is not very well understood. Many

parameters can influence the efficiency of the annealing, the temperature level is

particularly crucial. Usually a greater temperature allows a faster recovery. The risk,

however, may be to damage the device with excessive heat. Many devices show

annealing at room temperature. The biasing also plays a key role during annealing. If the

rate of damage recovery from annealing is greater than the rate of damage creation (when









the device is turned off for example) it is possible to use annealing as a part of a

hardening method.



Dose rate effects. When a photon penetrates into silicon, it deposits energy along

its track. The energy required to create an electron -hole pair in silicon has an average of

3.6 eV. These charge carriers contribute in conduction and react like any other electron

or hole. The contribution of these charge carriers to the total current in a device is called

photocurrent. A dose rate of 1 Gy/s in silicon is equivalent to the creation of 4. 109

electron-hole pairs per jtm3 of silicon. This means that a photon dose rate of 1 Gy/s on 1

jtm3 of silicon produces of 644 pA of current. It is then obvious that the dose rate effects

for silicon devices are negligible in the large majority of cases. There may be a concern

for devices that work with very low current in a very high dose rate (i.e., video cameras).

In that case the dose rate is so high that total dose failures are rapidly reached. These

results are also true for all semiconductor devices like germanium and gallium arsenide

devices, but may vary in magnitude.



Technology Families



P-N Junction devices. P-N junctions are used in many applications such as

switching, rectification, voltage reference (zener diode) or in optoelectronics. P-n

junctions are naturally radiation tolerant. The effect of radiation comes from both

minority carrier lifetime reduction resulting from displacement damages in the bulk and

also from charge trapping in the oxide layer insulating the junction. The lifetime









reduction of minority charge carriers increases the reverse leakage current, increases the

forward voltage drop and modifies the breakdown voltage. These changes are very

limited and often negligible under a total dose of 1 kGy. These limited changes in the

junction characteristic often do not cause any trouble in their application.



Bipolar technology. A bipolar transistor consists of two p-n junctions put

together in a single device. Two combinations are possible: n-p-n or p-n-p with an oxide

layer insulating the device. The bipolar technology is known for its natural radiation

resistance often greater than 10 kGy. Two types of damages occur during irradiation.

First, particles create defects in the bulk of silicon by displacement damage. These

defects operate as recombination centers for minority carriers and shorten their lifetime.

The second origin of damages lies in the trapped charge in the oxide passivation layer.

These trapped charges introduce new interface states at the frontier SiO2-Si interface that

also decreases minority carrier lifetime and increases the junction leakage current. The

impact of oxide trapped charges on bipolar devices is much smaller than for MOSFETs

since the oxide is not an active part and because the surface doping is much greater for a

bipolar transistors than for MOSFETs. Both bulk and interface defects decrease the

overall gain and increases the leakage current. The influence of one effect over the other

depends on the photon energy, the type of silicon, temperature, bias and the transistor

geometry. Gain reduction affects linear integrated circuits while leakage current affects

digital circuits. Both displacement and ionization damages have been described earlier in

this text. It is found that the gain loss due to displacement damage strongly depends on

the base width. The thinner base region that is increasingly used in modem integrated









circuits increases the radiation hardness of bipolar circuits. The photon energy also plays

a role since different gain values are observed after Co-60 and x-ray irradiation for the

same total dose [85]. The defects due to charge trapping in the oxide strongly depend on

component batches and on manufacturers. Since the oxide layer is used as insulator, its

geometry and thickness are not critical parameters for manufacture and a wide range of

effects can be expected. The effects of interface traps on gain loss are more obvious at

low current because the surface recombination of carriers takes out an important fraction

of minority carriers. This occurs because low currents are often surface currents. It is

possible to avoid such problems by an appropriate doping. The leakage current is also an

effect of the charge build-up of the Si-SiO2 interface. The trapped charges form a surface

channel that conducts a small current between collector and base. The leakage current

strongly depends on the impurity concentration in the oxide and on the bias during

irradiation. The geometry (vertical or lateral) and the type of transistor (p-n-p or n-p-n)

seem to play a role in the radiation hardness of a bipolar device although the dispersion of

results makes it difficult to draw any conclusive ranking. The bias is also a key

parameter, although it sometimes can improve or worsen the radiation sensitivity.

The most important radiation behavior with bipolar technology is that not only the

total dose effects depend on the dose rate during irradiation but also that a high dose rate

sometimes gives smaller defects than a low dose rate. This simply means that a failure

dose is often underestimated by several orders of magnitude when the test is

accomplished with a higher dose rate than for the actual application. This phenomenon is

due to the time required for oxide trapped charges to slowly moves to the Si-SiO2

interface [86]. On the other hand, this effect is not found for all bipolar devices,









sometimes a high dose rate gives more damage than a low dose rate [87, 88]. The fact

that an important fraction of the traps remain in the oxide bulk and does not shift to the

interface complicates this effect. This explanation is consistent with the strong variation

of the dose rate impact on total dose damages with the oxide thickness. It is found that

the difference of effects between high dose rate and low dose rate irradiation is greatly

reduced by using a thinner oxide layer. MOS devices do not suffer from this effect

because of the small thickness of their oxide. A testing at very low dose and at the exact

dose rate conditions as the operational environments is often impractical and too costly.

There are two alternatives. The first one is to over-test the bipolar device. This means

that if a device is certified up to 100 kGy with a high dose rate, it may operate without

failure up to 1 kGy with a low dose rate. The difficulty is to find the most accurate

overtest factor that converts the high dose rate results to low dose rate conditions. This

method may be very inaccurate when testing a complex electronic board because of the

complicated way components interact, and because the non-linear relation between the

high and low dose rates. A high dose rate can give an overestimated radiation resistance

for a MOS devices but underestimated results for a bipolar devices. The second

alternative is to irradiate the device under test at elevated temperature with a medium

dose rate [89]. It is found that an elevated temperature accelerates the shift of trapped

charges to the interface and gives closer results to a low dose rate exposure in a shorter

time. The limitation of this method is that an excessive temperature is needed when

working at high dose rate. A medium dose rate and a longer exposure time are not

avoidable. Secondly, it may be difficult to maintain a uniform temperature for a circuit

board under test. If a board under test includes MOS devices, the heat heals their









damages under radiation and an unexpected failure may occur in real application with

low dose rate.

A fraction of the damages can be recovered after exposure by the appropriate

annealing. A room temperature annealing however shows little recovery rate. This is

because the energy levels of the defects are important and are stable at room temperature.

An elevated temperature is required to anneal the device: between 1000C and 2000C to

suppress the interface states and between 1500C and 3000C for the trapped charges. The

highest temperature is needed to remove the bulk damages that result from displacement

of atoms: between 2000C and 4500C.



JFET. Junction field effect transistors (JFET) have strong radiation tolerance that

is even greater than bipolar devices. Unfortunately, their use is not very common

because of their limited power and voltage. Their low noise and high input impedance

make them very useful for pre-amplification of signals. Unlike the metal oxide

semiconductor FET, the current flow is not controlled through an oxide insulator but

directly on the surface of the semiconductor. The charge trapping that is characteristic of

damages in MOSFETs is then avoided. JFETs are also majority carrier devices that are

not affected by minority carrier lifetime reduction as with bipolar transistors. Radiation

slightly affects the parameters of JFETs such as transconductance, pinch-off, and on-

resistance. The most sensitive parameter is the gate-to-source leakage that can increase

after a total dose of 10 kGy. This may become a problem for low leakage applications

after a few kGy. The noise characteristic is often the reason for using a JFET, this noise

voltage rises slowly with integrated dose integrated dose [1, 90]. JFETs using Gallium









Arsenide materials are extremely resistant to radiation, even more than their silicon

counterpart (see page 108).



MOS technology. Unhardened MOS (Metal oxide semiconductor) devices are

very radiation sensitive. Their tolerance is usually less than 100 Gy and their use is often

to be avoided in radiation environments. Radiation effects on MOS devices are mainly

due to ionization damages. Most of the damages occur in the SiO2 insulator between the

gate and the channel. Radiation creates electron-hole pairs in the oxide. The electric

field across the oxide separates electrons and holes. The electron moves rapidly out of

the oxide due to its higher mobility, the hole drifts slowly in the oxide and both may be

trapped in defects. The generation of electron hole pairs from irradiation therefore results

in a build up of charges in the oxide. The consequence is that a higher gate voltage is

needed to produce the same electric field across the oxide. This results in a shift AV in

the drain current gate voltage characteristic of the device. Two types of traps generate

two different effects. The contribution from long-lived deep trapping of holes near the

Si-S02 interface give rise to a total positive charge Qot that causes a simple translation in

the current-voltage characteristic. The defects at the interfaces and their resulting trapped

charge Qit create a voltage shift but also a distortion of the current-voltage curve (see

Figure 3 and [91]).









Log (Ids)

N -channel -' ''


...... .... ./ /.


........ Pre-irradiation
-- 20 Gy
----- 80 Gy
------------------ 8 0 G y




V, (V)
-1 0 1 2

Figure 3: Radiation effects on n-channel MOS devices. Drain current versus gate voltage
for several total doses for a n-channel MOSFET.


For a p-channel MOS device, Qot and QA are both positive when the gate voltage

is at the threshold voltage. These charges in the insulator weaken the effects of the

electric field in the oxide. A lower voltage is required to get the pre-irradiation result.

This modification of threshold voltage increases continuously with the accumulated dose

and makes it impossible to turn-on the device. For an n-channel device, Qit is negative

when the gate voltage is at the threshold voltage. This means that the trapped hole charge

Qot and the interface charge Qt have opposite contributions that contribute to a complex

behavior. The effect of Qot far an n-channel device is the same as in a p-channel MOS; it

results in a lowered threshold voltage. The contribution of Qit may shows up at low

radiation dose rate and/or without biasing. The decrease of oxide charges Qot due to









annealing at room temperature allows the negatives charge QA to actually increase the

threshold voltage. This reaction is called "turnaround" and is due to the competition of

several processes. The build-up of Qot is a faster process than the creation of Qit. The

positive charges are created faster than negative charges, resulting in a positive total

charge. The low dose rate provides more time for annealing of trapped holes, the value

of Qot becomes comparable to QA, because charge creation and annealing takes more

time. The total charge in the oxide then goes from positive to increasingly negative. The

result of the low dose rate behavior is an increased voltage threshold. The turnaround is a

decrease followed by an increase of the threshold voltage. If this succession of threshold

voltage changes remains within acceptable limits, the radiation tolerance of components

may be extended by the turnaround effect. If a device is tested with a high dose rate and

its design is modified to adapt to the lower threshold voltage, then an exposure to a low

dose rate that results in an increase of a voltage threshold may lead to unexpected failure.

The shift in voltage threshold for p-channel and n-channel MOS devices affects

the logic compatibility of a system: logic 1 can be taken for logic 0 and vice-versa. The

voltage threshold change is not the only effect of radiation on a MOS device. Another

effect of gamma radiation is the growth of quiescent current (supply current when the

gate is not changing state). This increased leakage current is the result of the voltage

shift in n-channel devices. With n-channel MOS, no drain current exists initially for

Vg=0 V. After irradiation, the negative voltage shift allows an increasing amount of

drain-current to pass in the channel even at Vg=0 V. The result is a greater supply

current consumption of a MOS circuit. If the current exceeds the maximum output of the

power supply, it may create a failure and/or a destruction of power components. The









measurement of the supply current allows monitoring and prediction of the degradation

on MOS devices. The leakage current that exists in a transistor that is supposedly "off'

reduces the potential difference across its channel and reduces the difference between the

"on" and "off' states. The problem of logic compatibility is one of the main origins of

failure in MOS devices. The change in the slope of the ld-Vg characteristic results in a

reduced trans-conductance of the device. The smaller current drive increases switching

time and delays since it takes more time to accumulate charges to change the logical state

of a device [92]. This effect results in logic timing errors that limit the operational

frequency of a device. A microprocessor may fail at its nominal clock frequency, but can

operate properly at a lower frequency. Finally, the oxide located between the gate and

the channel is not the only source of problems. All modem MOS devices use thick oxide

layers as insulators to protect the active zones from interacting with the connectors or the

package. This zone of the component is the location of a parasitic parallel transistor near

the so called bird's beak region of the lateral oxide insulator and is due to trapped holes

in the oxide layer. Radiation generates leaks in this oxide that may shunt two conducting

elements and produce a failure earlier than other effects at the gate. Displacement

damages exists in MOS devices but are always swamped by the effects of ionization.

Displacement damages decrease the minority carrier conduction but do not affect MOS

components that are majority carrier devices.

The biasing of a device plays a major role in the radiation damage in MOS

devices. When a voltage is applied to the gate during irradiation, the electron-hole pairs

generated by the gamma rays are swept away by the intense electric field in the oxide. It

is improbable that an electron and a hole recombine. When the device is not powered, no




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